Algorithm to Find the Winner on a Tic Tac Toe Game

Tic-tac-toe is played by two players A and B on a 3 x 3 grid.

Here are the rules of Tic-Tac-Toe:

• Players take turns placing characters into empty squares (” “).
• The first player A always places “X” characters, while the second player B always places “O” characters.
• “X” and “O” characters are always placed into empty squares, never on filled ones.
• The game ends when there are 3 of the same (non-empty) character filling any row, column, or diagonal.
• The game also ends if all squares are non-empty.
• No more moves can be played if the game is over.

Given an array moves where each element is another array of size 2 corresponding to the row and column of the grid where they mark their respective character in the order in which A and B play. Return the winner of the game if it exists (A or B), in case the game ends in a draw return “Draw”, if there are still movements to play return “Pending”.

You can assume that moves is valid (It follows the rules of Tic-Tac-Toe), the grid is initially empty and A will play first.

Example 1:
Input: moves = [[0,0],[2,0],[1,1],[2,1],[2,2]]
Output: “A”

Explanation: “A” wins, he always plays first.

"X  "    "X  "    "X  "    "X  "    "X  "
"   " -> "   " -> " X " -> " X " -> " X "
"   "    "O  "    "O  "    "OO "    "OOX"

Example 2:
Input: moves = [[0,0],[1,1],[0,1],[0,2],[1,0],[2,0]]
Output: “B”
Explanation: “B” wins.

"X  "    "X  "    "XX "    "XXO"    "XXO"    "XXO"
"   " -> " O " -> " O " -> " O " -> "XO " -> "XO " 
"   "    "   "    "   "    "   "    "   "    "O  "

Example 3:

Input: moves = [[0,0],[1,1],[2,0],[1,0],[1,2],[2,1],[0,1],[0,2],[2,2]]
Output: “Draw”
Explanation: The game ends in a draw since there are no moves to make.

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"XXO" "OOX" "XOX"

"XXO"
"OOX"
"XOX"

Example 4:

Input: moves = [[0,0],[1,1]]
Output: “Pending”
Explanation: The game has not finished yet.

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"X  " " O " "   "

"X  "
" O "
"   "

Constraints:
1 <= moves.length <= 9
moves[i].length == 2
0 <= moves[i][j] <= 2
There are no repeated elements on moves.
moves follow the rules of tic tac toe.

Hints:
It’s straightforward to check if A or B won or not, check for each row/column/diag if all the three are the same.
Then if no one wins, the game is a draw if and only if the board is full, i.e. moves.length = 9 otherwise is pending.

C++ function to determine the winner of a Tic-Tac-Toe Game

Just check (bruteforce) if there are 3 connected pieces: 3 rows, 3 columns and 2 diagonals:

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bool win(char board[3][3], string &result) {
     for (int i = 0; i < 3; ++ i) {
        if ((board[i][0] != 0) && (board[i][0] == board[i][1]) 
                        && (board[i][1] == board[i][2])) {
            result = board[i][0];
            return true;
        }
        if ((board[0][i] != 0) && (board[0][i] == board[1][i]) 
                        && (board[1][i] == board[2][i])) {
            result = board[0][i];
            return true;
        }            
    }
    if ((board[1][1] != 0) && (board[0][0] == board[1][1]) 
                        && (board[1][1] == board[2][2])) {
        result = board[1][1];
        return true;
    }
    if ((board[1][1] != 0) && (board[0][2] == board[1][1]) 
                        && (board[1][1] == board[2][0])) {
        result = board[1][1];
        return true;
    }        
    return false;
}

bool win(char board[3][3], string &result) {
     for (int i = 0; i < 3; ++ i) {
        if ((board[i][0] != 0) && (board[i][0] == board[i][1]) 
                        && (board[i][1] == board[i][2])) {
            result = board[i][0];
            return true;
        }
        if ((board[0][i] != 0) && (board[0][i] == board[1][i]) 
                        && (board[1][i] == board[2][i])) {
            result = board[0][i];
            return true;
        }            
    }
    if ((board[1][1] != 0) && (board[0][0] == board[1][1]) 
                        && (board[1][1] == board[2][2])) {
        result = board[1][1];
        return true;
    }
    if ((board[1][1] != 0) && (board[0][2] == board[1][1]) 
                        && (board[1][1] == board[2][0])) {
        result = board[1][1];
        return true;
    }        
    return false;
}

The board has been initialized to 3×3 matrix with zero values:

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char board[3][3] = {};

char board[3][3] = {};

Alternatively, we could use the std::fill():

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std::fill(&board[0][0], &board[0][0] + sizeof(board), 0);

std::fill(&board[0][0], &board[0][0] + sizeof(board), 0);

Find the Winner on a Tic Tac Toe Game

A special case is to check if there is room for next play – either it is Draw or Pending if there is no winner.

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string tictactoe(vector<vector<int>>& moves) {
    char board[3][3] = {};
    //std::fill(&board[0][0], &board[0][0] + sizeof(board), ' ');
    string result;
    int i = 0;
    for (const auto &n: moves) {
        board[n[0]][n[1]] = ((i ++) % 2 == 0) ? 'A' : 'B';
        if (win(board, result)) {
            return result;
        }
    }
    return i == 9 ? "Draw" : "Pending";
}

string tictactoe(vector<vector<int>>& moves) {
    char board[3][3] = {};
    //std::fill(&board[0][0], &board[0][0] + sizeof(board), ' ');
    string result;
    int i = 0;
    for (const auto &n: moves) {
        board[n[0]][n[1]] = ((i ++) % 2 == 0) ? 'A' : 'B';
        if (win(board, result)) {
            return result;
        }
    }
    return i == 9 ? "Draw" : "Pending";
}

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