Find the kth smallest numbers in an unsorted integer array.

Solution: quick select

    public int kthSmallest(int k, int[] nums) {
        if(nums == null || nums.length == 0 || k < 1){
            return -1;
        }
        int start = 0, end = nums.length - 1, pos = 0;
        while(start >= 0 && end < nums.length){
            pos = partition(nums, start, end);
            if(pos + 1 == k){
                return nums[pos];
            } else if(pos + 1 < k){
                start = pos + 1;
            } else{
                end = pos - 1;
            }
        }
        return nums[pos];
    }
    private int partition(int[] nums, int left, int right){
        int l = left, r = right, pivot = nums[left];
        while(l < r){
            while(l < r && nums[r] >= pivot){
                r--;
            }
            nums[l] = nums[r];
            while(l < r && nums[l] <= pivot){
                l++;
            }
            nums[r] = nums[l];
        }
        nums[l] = pivot;
        return l;
    }

.or:

    public int kthLargestElement(int k, int[] nums) {
        if(nums == null || nums.length < k || k < 1){
            return 0;
        }
        return helper(nums, 0, nums.length - 1, k);
    }

    private int helper(int[] nums, int l, int r, int k){
        int left = l, right = r;
        int pivot = nums[left];
        while(left < right){
            while(left < right && nums[right] >= pivot){
                right--;
            }
            nums[left] = nums[right];
            while(left < right && nums[left] <= pivot){
                left++;
            }
            nums[right] = nums[left];
        }
        nums[left] = pivot;
        if(left == nums.length - k){
            return nums[left];
        } else if(left < nums.length - k){
            return helper(nums, left + 1, r, k);
        } else{
            return helper(nums, l, left - 1, k);
        }
    }