Kth Smallest Numbers in Unsorted Array（分别使用快排、归并、快选三种方法）

Find the kth smallest numbers in an unsorted integer array.

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Example
Given [3, 4, 1, 2, 5], k = 3, the 3rd smallest numbers are [1, 2, 3].

快排

public class Solution {    /*     * @param k: An integer     * @param nums: An integer array     * @return: kth smallest element     */    public int kthSmallest(int k, int[] nums) {        // write your code here        if (nums == null || nums.length == 0 || nums.length < k) {            return -1;        }        quickSort(nums, 0, nums.length - 1);        return nums[k - 1];    }    private void quickSort(int[] nums, int start, int end) {        if (start >= end) {            return;        }        int left = start;        int right = end;        int pivot = nums[(left + right) / 2];        while (left <= right) {            while (left <= right && nums[left] < pivot) {                left++;            }            while (left <= right && nums[right] > pivot) {                right--;            }            if (left <= right) {                int temp = nums[left];                nums[left] = nums[right];                nums[right] = temp;                left++;                right--;            }        }        quickSort(nums, start, right);        quickSort(nums, left, end);    }}

归并

public class Solution {    /*     * @param k: An integer     * @param nums: An integer array     * @return: kth smallest element     */    public int kthSmallest(int k, int[] nums) {        // write your code here        if (nums == null || nums.length == 0 || nums.length < k || k <= 0) {            return -1;        }        int[] val = new int[nums.length];        mergeSort(nums, val, 0, nums.length - 1);        return nums[k - 1];    }    private void mergeSort(int[] nums, int[] val, int start, int end) {        if (start >= end) {            return;        }        int mid = (start + end) / 2;        mergeSort(nums, val, start, mid);        mergeSort(nums, val, mid + 1, end);        merge(nums, val, start, mid, end);    }    private void merge(int[] nums, int[] val, int start, int mid, int end) {        int left = start;        int right = mid + 1;        int index = start;        while (left <= mid && right <= end) {            if (nums[left] < nums[right]) {                val[index++] = nums[left++];            } else {                val[index++] = nums[right++];            }        }        while (left <= mid) {            val[index++] = nums[left++];        }        while (right <= end) {            val[index++] = nums[right++];        }        for (int i = start; i <= end; i++) {            nums[i] = val[i];        }    }}

快选（最优解）

public class Solution {    /*     * @param k: An integer     * @param nums: An integer array     * @return: kth smallest element     */    public int kthSmallest(int k, int[] nums) {        // write your code here        if (nums == null || nums.length == 0 || nums.length < k) {            return -1;        }        quickSelect(nums, 0, nums.length - 1, k - 1);        return nums[k - 1];    }    private void quickSelect(int[] nums, int start, int end, int k) {        if (start >= end) {            return;        }        int left = start;        int right = end;        int pivot = nums[(left + right) / 2];        while (left <= right) {            while (left <= right && nums[left] < pivot) {                left++;            }            while (left <= right && nums[right] > pivot) {                right--;            }            if (left <= right) {                int temp = nums[left];                nums[left] = nums[right];                nums[right] = temp;                left++;                right--;            }        }        if (right >= k) {            quickSelect(nums, start, right, k);        } else {            quickSelect(nums, left, end, k);        }    }}