/*
* @lc app=leetcode.cn id=215 lang=cpp
*
* [215] 数组中的第K个最大元素
*/
// @lc code=start
class Solution {
public:
int findKthLargest(vector<int>& nums, int k) {
srand(time(0));
int n = nums.size();
int x = nums[0];
int ans = quickSelect(nums,0,n-1,n-k);
return ans;
}
//在[l,r]中找到第index小的数
int quickSelect(vector<int> &nums,int l,int r,int index) {
rand_pos(nums,l,r);
int pos = partition(nums,l,r);
if(pos == index) {
return nums[pos];
}
if(pos > index) {
return quickSelect(nums,l,pos-1,index);
}
else if(pos < index){
return quickSelect(nums,pos+1,r,index);
}
return 0;
}
//找到l-r里的一个随机元素，并和nums[r]替换
//返回大于nums[r]的元素下标
int rand_pos(vector<int> &nums,int l,int r) {
int len = r-l+1;
int rand_pos = l + (rand() % (len));
swap(nums[rand_pos],nums[r]);
return 0;
}
//把[l,r]里面小于nums[r]的元素放在前面
//返回大于nums[r]的元素下标
int partition(vector<int> &nums,int l,int r) {
int x = nums[r];
int j = l-1;
for(int i=l;i<r;i++) {
if(nums[i] <= x) {
swap(nums[++j],nums[i]);
}
}
swap(nums[j+1],nums[r]);
return j+1;
}
};
// @lc code=end
