299. Bulls and Cows

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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