二分

C++ 代码

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