chen-w
8/14/2017 - 5:41 AM

## quick select

quick select

``````class Solution {
/*
* @param k : description of k
* @param nums : array of nums
* @return: description of return
*/
public int kthLargestElement(int k, int[] nums) {
if (nums == null || nums.length == 0) {
return 0;
}
if (k <= 0) {
return 0;
}
return helper(nums, 0, nums.length - 1, nums.length - k + 1);

}
public int helper(int[] nums, int l, int r, int k) {
if (l == r) {
return nums[l];
}
int position = partition(nums, l, r);
if (position + 1 == k) {
return nums[position];
} else if (position + 1 < k) {
return helper(nums, position + 1, r, k);
}  else {
return helper(nums, l, position - 1, k);
}
}
public int partition(int[] nums, int l, int r) {
// 初始化左右指针和pivot
int left = l, right = r;
int pivot = nums[left];

// 进行partition
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];
}

// 返还pivot点到数组里面
nums[left] = pivot;
return left;
}
};``````