215. Kth Largest Element in an Array

Problem

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Example 1:

Input: [3,2,1,5,6,4] and k = 2
Output: 5

Example 2:

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4

**Note: ** You may assume k is always valid, 1 ≤ k ≤ array's length.

Solution

/**
 * @param {number[]} nums
 * @param {number} k
 * @return {number}
 */
var findKthLargest = function(nums, k) {
  return quickSelect(nums, 0, nums.length - 1, k);
};

var quickSelect = function (nums, left, right, k) {
  var le = left;
  var ri = right;
  var mid = nums[right];
  while (le < ri) {
    if (nums[le++] > mid) swap(nums, --le, --ri);
  }
  swap(nums, le, right);
  var len = right - le;
  if (len === k - 1) return nums[le];
  else if (len < k - 1) return quickSelect(nums, left, le - 1, k - len - 1);
  else return quickSelect(nums, le + 1, right, k);
};

var swap = function (nums, i, j) {
  var tmp = nums[i];
  nums[i] = nums[j];
  nums[j] = tmp;
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).