singleNumber , ex: [4,1,2,1,2]

var singleNumber = function(nums) {

    var ans = 0;

    for (var i = 0; i < nums.length; i++) {

        ans = ans ^ nums[i];

    }

    return ans;

};

example1:

Input: nums = [4,1,2,1,2]

Output: 4