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.

解題思路：

用歸併排序把數組重新由大到小排個序，然後返回第k大個數就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];
    }
};