TOP-k问题快排思想

package 剑指offer;

public class Num40最小的k个数快速选择解法 {
    public int[] getLeastNumbers(int[] arr, int k) {
        helper(arr, 0, arr.length - 1, k);
        int[] ans = new int[k];
        for (int i = 0; i < k; i++) {
            ans[i] = arr[i];
        }
        return ans;
    }

    public void helper(int[] arr, int left, int right, int k) {
        int index = partition(arr, left, right);
        if (index == k) {
            return;
        } else if (index > k) {
            helper(arr, left, index - 1, k);
        } else {
            helper(arr, index + 1, right,k);
        }
    }

    public int partition(int[] arr, int left, int right) {
        if (left > right) {
            return left;
        }
        int i = left;
        int j = right;
        int temp = 0;
        int base = arr[left];
        while (i < j) {
            while (i < j && arr[j] >= base) {
                j--;
            }
            while (i < j && arr[i] <= base) {
                i++;
            }
            if (i < j) {
                temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
        arr[left] = arr[i];
        arr[i] = base;
        return i;
    }
}