数据结构笔记总结(7.7)Leetcode上优先队列相关问题

347. 前K个高频元素

给定一个非空的整数数组,返回其中出现频率前 k 高的元素。

例如,

给定数组 [1,1,1,2,2,3] , 和 k = 2,返回 [1,2]

注意:

  • 你可以假设给定的 k 总是合理的,1 ≤ k ≤ 数组中不相同的元素的个数。
  • 你的算法的时间复杂度必须优于 O(n log n) , n 是数组的大小。

代码演示

使用我们自己的PriorityQueue

算法用的是我们自己的PriorityQueue,所以要把所有相关类做成内部类

import java.util.LinkedList;
import java.util.List;
import java.util.TreeMap;

class Solution {

    private class Array {

        private E[] data;
        private int size;

        // 构造函数,传入数组的容量capacity构造Array
        public Array(int capacity){
            data = (E[])new Object[capacity];
            size = 0;
        }

        // 无参数的构造函数,默认数组的容量capacity=10
        public Array(){
            this(10);
        }

        public Array(E[] arr){
            data = (E[])new Object[arr.length];
            for(int i = 0 ; i < arr.length ; i ++)
                data[i] = arr[i];
            size = arr.length;
        }

        // 获取数组的容量
        public int getCapacity(){
            return data.length;
        }

        // 获取数组中的元素个数
        public int getSize(){
            return size;
        }

        // 返回数组是否为空
        public boolean isEmpty(){
            return size == 0;
        }

        // 在index索引的位置插入一个新元素e
        public void add(int index, E e){

            if(index < 0 || index > size)
                throw new IllegalArgumentException("Add failed. Require index >= 0 and index <= size.");

            if(size == data.length)
                resize(2 * data.length);

            for(int i = size - 1; i >= index ; i --)
                data[i + 1] = data[i];

            data[index] = e;

            size ++;
        }

        // 向所有元素后添加一个新元素
        public void addLast(E e){
            add(size, e);
        }

        // 在所有元素前添加一个新元素
        public void addFirst(E e){
            add(0, e);
        }

        // 获取index索引位置的元素
        public E get(int index){
            if(index < 0 || index >= size)
                throw new IllegalArgumentException("Get failed. Index is illegal.");
            return data[index];
        }

        // 修改index索引位置的元素为e
        public void set(int index, E e){
            if(index < 0 || index >= size)
                throw new IllegalArgumentException("Set failed. Index is illegal.");
            data[index] = e;
        }

        // 查找数组中是否有元素e
        public boolean contains(E e){
            for(int i = 0 ; i < size ; i ++){
                if(data[i].equals(e))
                    return true;
            }
            return false;
        }

        // 查找数组中元素e所在的索引,如果不存在元素e,则返回-1
        public int find(E e){
            for(int i = 0 ; i < size ; i ++){
                if(data[i].equals(e))
                    return i;
            }
            return -1;
        }

        // 从数组中删除index位置的元素, 返回删除的元素
        public E remove(int index){
            if(index < 0 || index >= size)
                throw new IllegalArgumentException("Remove failed. Index is illegal.");

            E ret = data[index];
            for(int i = index + 1 ; i < size ; i ++)
                data[i - 1] = data[i];
            size --;
            data[size] = null; // loitering objects != memory leak

            if(size == data.length / 4 && data.length / 2 != 0)
                resize(data.length / 2);
            return ret;
        }

        // 从数组中删除第一个元素, 返回删除的元素
        public E removeFirst(){
            return remove(0);
        }

        // 从数组中删除最后一个元素, 返回删除的元素
        public E removeLast(){
            return remove(size - 1);
        }

        // 从数组中删除元素e
        public void removeElement(E e){
            int index = find(e);
            if(index != -1)
                remove(index);
        }

        public void swap(int i, int j){

            if(i < 0 || i >= size || j < 0 || j >= size)
                throw new IllegalArgumentException("Index is illegal.");

            E t = data[i];
            data[i] = data[j];
            data[j] = t;
        }

        @Override
        public String toString(){

            StringBuilder res = new StringBuilder();
            res.append(String.format("Array: size = %d , capacity = %d\n", size, data.length));
            res.append('[');
            for(int i = 0 ; i < size ; i ++){
                res.append(data[i]);
                if(i != size - 1)
                    res.append(", ");
            }
            res.append(']');
            return res.toString();
        }

        // 将数组空间的容量变成newCapacity大小
        private void resize(int newCapacity){

            E[] newData = (E[])new Object[newCapacity];
            for(int i = 0 ; i < size ; i ++)
                newData[i] = data[i];
            data = newData;
        }
    }

    private class MaxHeap> {

        private Array data;

        public MaxHeap(int capacity){
            data = new Array<>(capacity);
        }

        public MaxHeap(){
            data = new Array<>();
        }

        public MaxHeap(E[] arr){
            data = new Array<>(arr);
            for(int i = parent(arr.length - 1) ; i >= 0 ; i --)
                siftDown(i);
        }

        // 返回堆中的元素个数
        public int size(){
            return data.getSize();
        }

        // 返回一个布尔值, 表示堆中是否为空
        public boolean isEmpty(){
            return data.isEmpty();
        }

        // 返回完全二叉树的数组表示中,一个索引所表示的元素的父亲节点的索引
        private int parent(int index){
            if(index == 0)
                throw new IllegalArgumentException("index-0 doesn't have parent.");
            return (index - 1) / 2;
        }

        // 返回完全二叉树的数组表示中,一个索引所表示的元素的左孩子节点的索引
        private int leftChild(int index){
            return index * 2 + 1;
        }

        // 返回完全二叉树的数组表示中,一个索引所表示的元素的右孩子节点的索引
        private int rightChild(int index){
            return index * 2 + 2;
        }

        // 向堆中添加元素
        public void add(E e){
            data.addLast(e);
            siftUp(data.getSize() - 1);
        }

        private void siftUp(int k){

            while(k > 0 && data.get(parent(k)).compareTo(data.get(k)) < 0 ){
                data.swap(k, parent(k));
                k = parent(k);
            }
        }

        // 看堆中的最大元素
        public E findMax(){
            if(data.getSize() == 0)
                throw new IllegalArgumentException("Can not findMax when heap is empty.");
            return data.get(0);
        }

        // 取出堆中最大元素
        public E extractMax(){

            E ret = findMax();

            data.swap(0, data.getSize() - 1);
            data.removeLast();
            siftDown(0);

            return ret;
        }

        private void siftDown(int k){

            while(leftChild(k) < data.getSize()){
                int j = leftChild(k); // 在此轮循环中,data[k]和data[j]交换位置
                if( j + 1 < data.getSize() &&
                        data.get(j + 1).compareTo(data.get(j)) > 0 )
                    j ++;
                // data[j] 是 leftChild 和 rightChild 中的最大值

                if(data.get(k).compareTo(data.get(j)) >= 0 )
                    break;

                data.swap(k, j);
                k = j;
            }
        }

        // 取出堆中的最大元素,并且替换成元素e
        public E replace(E e){

            E ret = findMax();
            data.set(0, e);
            siftDown(0);
            return ret;
        }
    }

    private interface Queue {

        int getSize();
        boolean isEmpty();
        void enqueue(E e);
        E dequeue();
        E getFront();
    }

    private class PriorityQueue> implements Queue {

        private MaxHeap maxHeap;

        public PriorityQueue(){
            maxHeap = new MaxHeap<>();
        }

        @Override
        public int getSize(){
            return maxHeap.size();
        }

        @Override
        public boolean isEmpty(){
            return maxHeap.isEmpty();
        }

        @Override
        public E getFront(){
            return maxHeap.findMax();
        }

        @Override
        public void enqueue(E e){
            maxHeap.add(e);
        }

        @Override
        public E dequeue(){
            return maxHeap.extractMax();
        }
    }

    private class Freq implements Comparable{

        public int e, freq;

        public Freq(int e, int freq){
            this.e = e;
            this.freq = freq;
        }

        @Override
        public int compareTo(Freq another){
            if(this.freq < another.freq)
                return 1;
            else if(this.freq > another.freq)
                return -1;
            else
                return 0;
        }
    }

    public List topKFrequent(int[] nums, int k) {

        TreeMap map = new TreeMap<>();
        for(int num: nums){
            if(map.containsKey(num))
                map.put(num, map.get(num) + 1);
            else
                map.put(num, 1);
        }

        PriorityQueue pq = new PriorityQueue<>();
        for(int key: map.keySet()){
            if(pq.getSize() < k)
                pq.enqueue(new Freq(key, map.get(key)));
            else if(map.get(key) > pq.getFront().freq){
                pq.dequeue();
                pq.enqueue(new Freq(key, map.get(key)));
            }
        }

        LinkedList res = new LinkedList<>();
        while(!pq.isEmpty())
            res.add(pq.dequeue().e);
        return res;
    }

    private static void printList(List nums){
        for(Integer num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {1, 1, 1, 2, 2, 3};
        int k = 2;
        printList((new Solution()).topKFrequent(nums, k));
    }
}

使用Java自己的优先队列

代码1

import java.util.LinkedList;
import java.util.List;
import java.util.PriorityQueue;
import java.util.TreeMap;

public class Solution {

    private class Freq implements Comparable{

        public int e, freq;

        public Freq(int e, int freq){
            this.e = e;
            this.freq = freq;
        }

        public int compareTo(Freq another){
            if(this.freq < another.freq)
                return -1;
            else if(this.freq > another.freq)
                return 1;
            else
                return 0;
        }
    }

    public List topKFrequent(int[] nums, int k) {

        TreeMap map = new TreeMap<>();
        for(int num: nums){
            if(map.containsKey(num))
                map.put(num, map.get(num) + 1);
            else
                map.put(num, 1);
        }

        PriorityQueue pq = new PriorityQueue<>();
        for(int key: map.keySet()){
            if(pq.size() < k)
                pq.add(new Freq(key, map.get(key)));
            else if(map.get(key) > pq.peek().freq){
                pq.remove();
                pq.add(new Freq(key, map.get(key)));
            }
        }

        LinkedList res = new LinkedList<>();
        while(!pq.isEmpty())
            res.add(pq.remove().e);
        return res;
    }

    private static void printList(List nums){
        for(Integer num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {1, 1, 1, 2, 2, 3};
        int k = 2;
        printList((new Solution()).topKFrequent(nums, k));
    }
}

代码2

import java.util.*;

public class Solution2 {

    private class Freq{

        public int e, freq;

        public Freq(int e, int freq){
            this.e = e;
            this.freq = freq;
        }
    }

    private class FreqComparator implements Comparator{

        @Override
        public int compare(Freq a, Freq b){
            return a.freq - b.freq;
        }
    }

    public List topKFrequent(int[] nums, int k) {

        TreeMap map = new TreeMap<>();
        for(int num: nums){
            if(map.containsKey(num))
                map.put(num, map.get(num) + 1);
            else
                map.put(num, 1);
        }

        PriorityQueue pq = new PriorityQueue<>(new FreqComparator());
        for(int key: map.keySet()){
            if(pq.size() < k)
                pq.add(new Freq(key, map.get(key)));
            else if(map.get(key) > pq.peek().freq){
                pq.remove();
                pq.add(new Freq(key, map.get(key)));
            }
        }

        LinkedList res = new LinkedList<>();
        while(!pq.isEmpty())
            res.add(pq.remove().e);
        return res;
    }

    private static void printList(List nums){
        for(Integer num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {1, 1, 1, 2, 2, 3};
        int k = 2;
        printList((new Solution()).topKFrequent(nums, k));
    }
}

代码3

import java.util.*;

public class Solution3 {

    private class Freq{

        public int e, freq;

        public Freq(int e, int freq){
            this.e = e;
            this.freq = freq;
        }
    }

    public List topKFrequent(int[] nums, int k) {

        TreeMap map = new TreeMap<>();
        for(int num: nums){
            if(map.containsKey(num))
                map.put(num, map.get(num) + 1);
            else
                map.put(num, 1);
        }

        PriorityQueue pq = new PriorityQueue<>(new Comparator() {
            @Override
            public int compare(Freq a, Freq b) {
                return a.freq - b.freq;
            }
        });
        for(int key: map.keySet()){
            if(pq.size() < k)
                pq.add(new Freq(key, map.get(key)));
            else if(map.get(key) > pq.peek().freq){
                pq.remove();
                pq.add(new Freq(key, map.get(key)));
            }
        }

        LinkedList res = new LinkedList<>();
        while(!pq.isEmpty())
            res.add(pq.remove().e);
        return res;
    }

    private static void printList(List nums){
        for(Integer num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {1, 1, 1, 2, 2, 3};
        int k = 2;
        printList((new Solution()).topKFrequent(nums, k));
    }
}

代码4

import java.util.*;

public class Solution4 {

    public List topKFrequent(int[] nums, int k) {

        TreeMap map = new TreeMap<>();
        for(int num: nums){
            if(map.containsKey(num))
                map.put(num, map.get(num) + 1);
            else
                map.put(num, 1);
        }

        PriorityQueue pq = new PriorityQueue<>(new Comparator() {
            @Override
            public int compare(Integer a, Integer b) {
                return map.get(a) - map.get(b);
            }
        });
        for(int key: map.keySet()){
            if(pq.size() < k)
                pq.add(key);
            else if(map.get(key) > map.get(pq.peek())){
                pq.remove();
                pq.add(key);
            }
        }

        LinkedList res = new LinkedList<>();
        while(!pq.isEmpty())
            res.add(pq.remove());
        return res;
    }

    private static void printList(List nums){
        for(Integer num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {1, 1, 1, 2, 2, 3};
        int k = 2;
        printList((new Solution()).topKFrequent(nums, k));
    }
}

代码5

import java.util.*;

public class Solution5 {

    public List topKFrequent(int[] nums, int k) {

        TreeMap map = new TreeMap<>();
        for(int num: nums){
            if(map.containsKey(num))
                map.put(num, map.get(num) + 1);
            else
                map.put(num, 1);
        }

        PriorityQueue pq = new PriorityQueue<>(
                (a, b) -> map.get(a) - map.get(b)
            );
        for(int key: map.keySet()){
            if(pq.size() < k)
                pq.add(key);
            else if(map.get(key) > map.get(pq.peek())){
                pq.remove();
                pq.add(key);
            }
        }

        LinkedList res = new LinkedList<>();
        while(!pq.isEmpty())
            res.add(pq.remove());
        return res;
    }

    private static void printList(List nums){
        for(Integer num: nums)
            System.out.print(num + " ");
        System.out.println();
    }

    public static void main(String[] args) {

        int[] nums = {1, 1, 1, 2, 2, 3};
        int k = 2;
        printList((new Solution()).topKFrequent(nums, k));
    }
}

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