[LeetCode][JavaScript]Missing Number

Missing Number

Given an array containing n distinct numbers taken from 0, 1, 2, ..., n, find the one that is missing from the array.

For example,
Given nums = [0, 1, 3] return 2.

Note:
Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?

https://leetcode.com/problems/missing-number/

---

求出序列中缺少的那个数，序列肯定是乱序的。

把0到n的数相加(等差数列求和)，再一个个减掉，最后剩下的就是结果。

本来以为相加可能超过了长度限制，要用大数加减法，试了一下并不用。

/**
 * @param {number[]} nums
 * @return {number}
 */
var missingNumber = function(nums) {
    var i, res = 0;
    res = parseInt((nums.length + 1) * nums.length / 2);
    for(i = 0; i < nums.length; i++){
        res -= nums[i];
    }
    return res;
};