快速排序和快速选择（quickSort and quickSelect）算法

排序算法：快速排序(quicksort)递归与非递归算法
TopK问题：快速选择(quickSelect)算法

import java.util.*;
import java.lang.*;public class Demo {// 非递归 using stackpublic static void quickSortStack(int[] nums, int left, int right) {if (left >= right) return;Stack<Range> stack = new Stack<Range>();stack.push( new Range(left, right) );while( !stack.empty() ) {Range curRange = stack.pop();if (curRange.left < curRange.right) {int pivotIdx = partition(nums, curRange.left, curRange.right);stack.push( new Range(curRange.left, pivotIdx - 1) );stack.push( new Range(pivotIdx + 1, curRange.right) );}}}// recursion quickSortpublic static void quickSort(int[] nums, int left, int right) {if (left < right) {int pIdx = partition(nums, left, right);quickSort(nums, left, pIdx - 1);quickSort(nums, pIdx + 1, right);} }public static int partition(int[] nums, int left, int right) {// nums[left]~nums[retIdx - 1] <= nums[retIdx] <= nums[retIdx ~ right]if (left == right) return left;int pivot = nums[left]; // mark the leftmost value as the pivot. and store it.int lp = left, rp = right;while(lp < rp) {while(lp < rp && nums[rp] >= pivot)rp--;nums[lp] = nums[rp]; //move the smaller value to left Range.while(lp < rp && nums[lp] <= pivot)lp++;nums[rp] = nums[lp]; // move the bigger value to right Range.}nums[lp] = pivot;return lp;}public static void swap(int[] nums, int i, int j) {int tmp = nums[i];nums[i] = nums[j];nums[j] = tmp;}public static int partitionV2(int[] nums, int left, int right) {int l = left;int r = right + 1;int pivot = nums[left];while(true) {while( l < right && nums[++l] < pivot )if ( l == right ) break;while( r > left && nums[--r] >= pivot )if ( r == left ) break;if (l >= r)break;swap(nums, l, r);}swap(nums, left, r);return r;}//TopK问题public static int findKthLargest(int[] nums, int k) {return quickSelect(nums, k, 0, nums.length - 1);}//快速选择算法public static int quickSelect(int[] nums, int k, int left, int right) {if (left == right) return nums[left];int index = partition(nums, left, right);if ( index - left + 1 == k) {return nums[index];}else if ( index - left + 1 > k ) {return quickSelect(nums, k, left, index - 1);}else {return quickSelect(nums, k - index + left - 1, index + 1, right);}}public static void showArrays(int[] nums, String str) {System.out.println("===" + str + "===");showArrays(nums);}public static void showArrays(int[] nums) {for (int i = 0; i < nums.length; i++)System.out.printf(nums[i] + " ");System.out.println();}public static void main(String[] args) {int[] nums = new int[] {1,3,2,3,4,2,7,5};// int[] nums = new int[]{3,2,3};showArrays(nums, "origin");//print i-th min nubmer.for (int i = 1; i <= nums.length; i++) {System.out.println( i + " " + findKthLargest(nums, i) );}quickSortStack(nums, 0, nums.length - 1);showArrays(nums, "quickSortStack");}
}class Range {public int left;public int right;public Range(int left, int right) {this.left = left;this.right = right;}
}