Find the kth smallest numbers in an unsorted integer array.

Have you met this question in a real interview?

Yes

Example

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

Challenge

An O(nlogn) algorithm is acceptable, if you can do it in O(n), that would be great.

class Solution {
public:
    /*
     * @param k an integer
     * @param nums an integer array
     * @return kth smallest element
     */
    int kthSmallest(int k, vector<int> nums) {
        // write your code here
        int start = 0, end = nums.size() - 1;
        while (true) {
            if (start > end) {
                break;
            }
            int i = start, j = start, l = end + 1;
            int pivot = nums[start + (end - start >> 1)];
            while (i < l) {
                if (nums[i] < pivot) {
                    swap(nums[i++], nums[j++]);
                } else if (nums[i] == pivot) {
                    ++i;
                } else {
                    swap(nums[i], nums[--l]);
                }
            }
            if (k - 1 < j) {
                end = j - 1;
            } else if (k - 1 >= l) {
                start = l;
            } else {
                return pivot;    
            }
        }
        return -1;
    }
};