1
var singleNonDuplicate = function (nums) {
2
  var start = 0;
3
  var end = nums.length - 1;
4
  // to check one element
5
  if (nums.length == 1) return nums[start];
6
  while (start <= end) {
7
    if (nums[start] != nums[start + 1]) {
8
      return nums[start];
9
    }
10

11
    if (nums[end] != nums[end - 1]) {
12
      return nums[end];
13
    }
14
    // increment two point
15
    start = start + 2;
16
    end = end - 2;
17
  }
18
};