剑指offer 53：0~n-1中缺失的数字

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