TopKProblem
TopKProblem.java 源码
package datastructure.heap.leetcode;
import java.util.Arrays;
import java.util.PriorityQueue;
import java.util.Queue;
/**
* @author roseduan
* @time 2019/11/16 15:17
* @description 使用快速排序的思想求top k，以及另外几种解法
*/
public class TopKProblem {
/**
* 求topK，使用类似快排的分区思想
*/
private static int topK1(int[] data, int k){
int length = data.length;
if (k > length){
throw new IllegalArgumentException("Parameter k is too large!");
}
return helper(data, 0, length - 1, k);
}
private static int helper(int[] data, int p, int r, int k){
int q = partition(data, p, r);
if (q + 1 == k){
return data[q];
}
else if (q + 1 < k){
return helper(data, q + 1, r, k);
}
else {
return helper(data, p, q - 1, k);
}
}
private static int partition(int[] data, int p, int r){
int i = p;
int pivot = data[r];
for (int j = p; j < r; j++) {
if (data[j] > pivot){
int temp = data[i];
data[i] = data[j];
data[j] = temp;
i++;
}
}
data[r] = data[i];
data[i] = pivot;
return i;
}
/**
* 求topK，使用排序
*/
private static int topK2(int[] data, int k){
int length = data.length;
if (k > length){
throw new IllegalArgumentException("Parameter k is too large!");
}
Arrays.sort(data);
return data[length - k];
}
/**
* 求topK，使用堆
*/
private static int topK3(int[] data, int k){
int length = data.length;
if (k > length){
throw new IllegalArgumentException("Parameter k is too large!");
}
Queue<Integer> queue = new PriorityQueue<>(k);
for (int i = 0; i < k; i++) {
queue.add(data[i]);
}
for (int i = k; i < length; i++) {
if (data[i] > queue.peek()){
queue.poll();
queue.add(data[i]);
}
}
return queue.peek();
}
}
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