leetcode-268

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?

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function  (nums) {
        var leng = nums.length;
        var sum = (leng+1)*leng/2;
        var numsSum=0;
        for(var i=0;i<leng;i++){
            numsSum+=nums[i];
        }
        return sum-numsSum;
    }