difficult：easy #561
Given an array of 2n integers, your task is to group these integers into n pairs of integer, say (a1, b1), (a2, b2), …, (an, bn) which makes sum of min(ai, bi) for all i from 1 to n as large as possible.

Example 1:
Input: [1,4,3,2]

Output: 4
Explanation: n is 2, and the maximum sum of pairs is 4 = min(1, 2) + min(3, 4).
Note:
n is a positive integer, which is in the range of [1, 10000].
All the integers in the array will be in the range of [-10000, 10000].

解法:

#include<vector>
void quicksort(std::vector<int> & array, int left, int right);
int arrayPairSum(std::vector<int>& nums) {
    int result = 0;
    quicksort(nums, 0, nums.size() - 1);
    for (int i = 0; i < nums.size(); i += 2) {
        result += nums[i];
    }
    return result;
}
void quicksort(std::vector<int> & array, int left, int right){
    int i, j, t, temp;
    if (left > right)
        return;
    temp = array[left];
    i = left;
    j = right;
    while (i != j){

        while (array[j] >= temp && i < j)
            j--;
        while (array[i] <= temp && i < j)
            i++;
        if (i<j) {
            t = array[i];
            array[i] = array[j];
            array[j] = t;
        }
    }
    array[left] = array[i];
    array[i] = temp;
    quicksort(array, left, i - 1);
    quicksort(array, i + 1, right);
}

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