Missing Number.js

/**
 * @param {number[]} nums
 * @return {number}
 */

function find(nums, v) {
    for (var j in nums) {
        if (v == nums[j]) {
            return true
        }
    }
    return false;
         
} 
 
var missingNumber = function(nums) {
    // sort
    /*
    var max = nums.length, i = 0;
    nums.sort(function(a,b){
        return a-b;
    });
    while(i<max) {
        if (nums[i] != i) {
            return i;
        }
        i++;
    }
    return nums.pop() + 1;
    */
    
    // o(n*n)
    /*
    var max = nums.length, i = 0;
    while(i<=max) {
        if (!find(nums, i)) {
            return i;
        }
        i++;
    }
    */
    
    // o(n)
    var len = nums.length;
    
    if (len == 1) {
        return 1 - nums[0];
    }
    
    sum = (1+len) * (len) / 2;
    
    for (var i in nums) {
        sum -= nums[i];
    }
    return sum;
    
};