数组中第K大的数

输入：array
,1<=k<=n-1

输出：第k大的数

思路：quick sort partition，O（n）

int partition(vector<int>&nums, int s, int e){
int idx = s + rand() % (e - s + 1);
swap(nums[idx], nums[e]);
int big = s - 1;
for (idx = s; idx<e; ++idx){
if (nums[idx]>nums[e]){
++big;
if (big != idx)
swap(nums[idx], nums[big]);
}
}
++big;
swap(nums[big], nums[e]);
return big;
}
int findKthLargest(vector<int>& nums, int k) {
int s = 0, e = nums.size() - 1;
int idx = partition(nums, s, e);
while (idx != k - 1){
if (idx>k - 1)
e = idx - 1;
else
s = idx + 1;
idx = partition(nums, s, e);
}
return  nums[idx];
}