215. Kth Largest Element in an Array

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.

class Solution {public:    int findKthLargest(vector<int>& nums, int k) {        arrangeRight(nums,0,nums.size()-1,k);//将前k个大数放在nums右边，O(n).        quickSort(nums,nums.size()-k,nums.size()-1); //对前k个数快排。O(klogk).        return nums[nums.size()-k];    }    void arrangeRight(vector<int>& nums, int start, int end, int k){    if(start >= end)    return;    int key = nums[start];    int i = start, j = end;    while(i!=j){    while(j>i && nums[j] >= key)    --j;    swap(nums,i,j);    while( i<j && nums[i] <= key)    ++i;    swap(nums,i,j);    }//a[i] = key    if(k == (nums.size() - i)) return;    if(k < (nums.size() - i)) arrangeRight(nums,i+1,end,k);    if(k > (nums.size() - i)) arrangeRight(nums,start,i-1,k-(nums.size()-i));    }        void quickSort(vector<int>& nums, int start, int end){    if(start >= end)    return;    int key = nums[start];    int i = start, j = end;    while(i!=j){    while(j>i && nums[j] >= key)    --j;    swap(nums,i,j);    while( i<j && nums[i] <= key)    ++i;    swap(nums,i,j);    }    quickSort(nums,start,i-1);    quickSort(nums,i+1,end);    }    void swap(vector<int> &v,int a,int b){        int t = v[a];        v[a] = v[b];        v[b] = t;    } };

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