215. Kth Largest Element in an Array

题目：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.

For example,

Given [3,2,1,5,6,4] and k = 2, return 5.

Note:

You may assume k is always valid, 1 ≤ k ≤ array’s length.

思路：这道题目最简单的方法就是排序，然后根据k输出对应下标位置的值。代码实现如下：

class Solution {
public:
int findKthLargest(vector<int>& nums, int k) {
sort(nums.begin(), nums.end());
return nums[nums.size()-k];
}
};

另一种方法就是没有必要将所有的元素排序，使用select algorithm，具体代码实现如下：

class Solution {
public:
int partition(vector<int>& nums, int lo, int hi) {
int num = nums[lo];

while (lo < hi) {
while (lo < hi && nums[hi] >= num) hi--;
if (lo < hi) swap(nums[hi], nums[lo]);
while (lo < hi && nums[lo] <= num) lo++;
if (lo < hi) swap(nums[hi], nums[lo]);
}
nums[lo] = num;
return lo;
}

int findKthLargest(vector<int>& nums, int k) {
k = nums.size()-k;
int lo = 0, hi = nums.size()-1;

while (lo < hi) {
int j = partition(nums, lo, hi);

if (j < k) lo = j+1;
else if (j > k) hi = j-1;
else break;
}
return nums[k];
}
};