Given an array of 2n integers, your task is to group these integers into n pairs of integer, say (a, b), (a, b), …, (a, b) which makes sum of min(a, b) for all i from 1 to n as large as possible.

    Example 1:

    1. Input: [1,4,3,2]
    3. Output: 4
    4. Explanation: n is 2, and the maximum sum of pairs is 4 = min(1, 2) + min(3, 4).

    Note:

    1. n is a positive integer, which is in the range of [1, 10000].

    2. All the integers in the array will be in the range of [-10000, 10000].

    /**
 * @param {number[]} nums
 * @return {number}
 */
var arrayPairSum = function(nums) {
    const hash = [];
    for (let i = 0; i < 20001; ++i) {
        hash[i] = 0;
    }
    let sum = 0;
    let min= Number.MAX_VALUE;
    let max= Number.MIN_VALUE;
    for (let i = 0; i < nums.length; ++i) {
        let cur = nums[i] + 10000;
        ++hash[cur];
        min = Math.min(min, cur);
        max = Math.max(max, cur);
    }
    let evenOdd = 0;
    for (let i = min; i <= max; ++i) {
        let curAmount = hash[i];
           for (let j = 0; j < curAmount; ++j) {
                if (evenOdd == 0) {
                    sum += i-10000;
                }  
                evenOdd ^= 1;
           } 
    }
    return sum; 
};