leetCode: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.

解题思路：

用归并排序把数组重新由大到小排个序，然后返回第k大个数就OK了。

class Solution {
public:
    void merge(vector<int>& nums, int L, int R){
        int mid = (L + R)/2;
        int* c = new int[R - L + 1];
        int i = L, j = mid + 1, k = 0;
        while(i <= mid && j <= R){
            if(nums[i] >= nums[j])c[k++] = nums[i++];
            else c[k++] = nums[j++];
        }
        while(i <= mid){
            c[k++] = nums[i++];
        }
        while(j <= R){
            c[k++] = nums[j++];
        }
        for(i = 0; i < k; i++){
            nums[L + i] = c[i];
        }
        delete[] c;
    }
    void mergeSort(vector<int>& nums, int L, int R){
        if(L >= R)return;
        int mid = (L + R)/2;
        mergeSort(nums, L, mid);
        mergeSort(nums, mid + 1, R);
        merge(nums, L, R);
    }
    int findKthLargest(vector<int>& nums, int k) {
        int len = nums.size();
        if(k > len)return -1;
        int mid = (len - 1)/2;
        mergeSort(nums, 0, mid);
        mergeSort(nums, mid + 1, len - 1);
        merge(nums, 0, len - 1);
        return nums[k - 1];
    }
};