Find Winner of Tic Tac Toe Game

Companies:
  • Amazon Interview Questions

Problem Source: LeetCode
Comapnies: Amazon

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:

  1. Players take turns placing characters into empty squares (” “).
  2. The first player A always places “X” characters, while the second player B always places “O” characters.
  3. “X” and “O” characters are always placed into empty squares, never on filled ones.
  4. The game ends when there are 3 of the same (non-empty) character filling any row, column, or diagonal.
  5. The game also ends if all squares are non-empty.
  6. 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],[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 “

Examples 2:

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 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.
“XXO”
“OOX”
“XOX”

Example 4:

Input: moves = [[0,0],[1,1]]

Output: “Pending”

Explanation: The game has not finished yet.
“X “
” O “
” “

Constraints:

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

Solution:

The implemetation for this solution is simple without using any data structure except for arrays. We will create a string array of size 3*3 and used it as our tic-tac-toe board.

Next we check the inputs given through moves[][] array and apply it to our board[][] matrix.

Now we will check for rows, columns and diagonals seperately.

Al the things when done, we will return our winner or “draw” or “pending” according to the given inputs.

See this below implementation to understand the whole process. The implementation of this problem will be long but it contains basic maipulation and techniques to solve the whole problem.

Implementation:

Java:

public class Solution {
        public String tictactoe(int[][] moves) {
            String[][] board = new String[3][3];
            for (int i = 0; i < moves.length; i++) {
                if (i % 2 == 0) {
                    board[moves[i][0]][moves[i][1]] = "X";
                } else {
                    board[moves[i][0]][moves[i][1]] = "O";
                }
                if (i > 3) {
                    if (!wins(board).equals("")) {
                        return wins(board);
                    }
                }
            }
            return moves.length == 9 ? "Draw" : "Pending";
        }

        private String wins(String[][] board) {
            //check rows
            for (int i = 0; i < 3; i++) {
                if (board[i][0] == null) {
                    break;
                }
                String str = board[i][0];
                if (str.equals(board[i][1]) && str.equals(board[i][2])) {
                    return getWinner(str);
                }
            }

            //check columns
            for (int j = 0; j < 3; j++) {
                if (board[0][j] == null) {
                    break;
                }
                String str = board[0][j];
                if (str.equals(board[1][j]) && str.equals(board[2][j])) {
                    return getWinner(str);
                }
            }

            //check diagonals
            if (board[1][1] == null) {
                return "";
            }
            String str = board[1][1];
            if (str.equals(board[0][0]) && str.equals(board[2][2]) || (str.equals(board[0][2]) && str.equals(board[2][0]))) {
                return getWinner(str);
            }
            return "";
        }

        private String getWinner(String str) {
            if (str.equals("X")) {
                return "A";
            } else {
                return "B";
            }
        }
    }
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