---
tags: leetcode
---

# [299. Bulls and Cows](https://leetcode.com/problems/bulls-and-cows/)
---
# My Solution
## The Key Idea for Solving This Coding Question
## C++ Code

#define MATCHED ('x')

class Solution {
public:
    string getHint(string secret, string guess) {
        vector<int> digitCnt(10, 0);
        int bullCnt = 0;
        for (int i = 0; i < secret.size(); ++i) {
            if (secret[i] == guess[i]) {
                ++bullCnt;
                guess[i] = MATCHED;
            } else {
                ++digitCnt[secret[i] - '0'];
            }
        }

        int cowCnt = 0;
        for (int i = 0; i < guess.size(); ++i) {
            if (guess[i] != MATCHED && digitCnt[guess[i] - '0'] > 0) {
                // guess[i] is in secret, but not located at the
                // correct position.
                ++cowCnt;
                --digitCnt[guess[i] - '0'];
            }
        }

        return to_string(bullCnt) + "A" + to_string(cowCnt) + "B";
    }
};

## Time Complexity
$O(n)$
$n$ is the length of `secret`.
## Space Complexity
$O(1)$
# Miscellane
<!--
# Test Cases

"1807"
"7810"

"1123"
"0111"

"1"
"0"

"1"
"1"

"1234"
"0111"

"11"
"10"

"1122"
"2211"

"1234"
"4321"

"1234"
"5678"

-->