剑指 Offer 53

class Solution {
public:
    int missingNumber(vector<int>& nums) {
        int n = nums.size() + 1;
        int l = 0, r = n - 1;
        while (l < r) {
            int mid = l + r >> 1;
            if (nums[mid] > mid) r = mid;
            else l = mid + 1;
        }

        return l;
    }
};