简单归并排序

问题：

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.

解析：

将数组内的元素排序后直接得出，考虑用归并排序。

答案：

class Solution {
public:
    int findKthLargest(vector& nums, int k) {
        int size = nums.size();
        merge_sort(nums, 0, size - 1);
        return nums[size - k];
    }
private:
    void merge(vector &nums, int p, int q, int r) {
        vector L1;
        for(int i = p; i < q + 1; i++) {
        L1.push_back(nums[i]);
}
        vector L2;
        for(int i = q + 1; i < r + 1; i++) {
        L2.push_back(nums[i]);
}
        L1.push_back(INT_MAX);
        L2.push_back(INT_MAX);


        int i = 0, j = 0;
        for(int k = p; k < r + 1; k++) {
            if(L1[i] < L2[j]) {
                nums[k] = L1[i];
                i++;
            }
            else {
                nums[k] = L2[j];
                j++;
            }
        } 
    }
    void merge_sort(vector &nums, int p, int r) {
        if(p < r) {
            int q = (p + r) / 2;
            merge_sort(nums, p, q);
            merge_sort(nums, q + 1, r);
            merge(nums, p, q, r);
        }
    }
};