794. Valid Tic-Tac-Toe State (Medium)
https://leetcode.com/problems/valid-tic-tac-toe-state/
A Tic-Tac-Toe board is given as a string array board
. Return True if and only if it is possible to reach this board position during the course of a valid tic-tac-toe game.
The board
is a 3 x 3 array, and consists of characters " "
, "X"
, and "O"
. The " " character represents an empty square.
Here are the rules of Tic-Tac-Toe:
- Players take turns placing characters into empty squares (" ").
- The first player always places "X" characters, while the second player always places "O" characters.
- "X" and "O" characters are always placed into empty squares, never 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.
Example 1: Input: board = ["O ", " ", " "] Output: false Explanation: The first player always plays "X". Example 2: Input: board = ["XOX", " X ", " "] Output: false Explanation: Players take turns making moves. Example 3: Input: board = ["XXX", " ", "OOO"] Output: false Example 4: Input: board = ["XOX", "O O", "XOX"] Output: true
Note:
board
is a length-3 array of strings, where each stringboard[i]
has length 3.- Each
board[i][j]
is a character in the set{" ", "X", "O"}
.
Solutions
class Solution {
// This is a nasty problem, too many special cases, skip it.
public boolean validTicTacToe(String[] board) {
int diff = count(board, 'X') - count(board, 'O');
// Initially, count of X must be equal to or 1 more than O, otherwise not a
// valid Tic-Tac-Toe board
if (diff > 1 || diff < 0) {
return false;
}
if (hasWon(board, 'X')) {
return diff == 1;
}
if (hasWon(board, 'O')) {
return diff == 0;
}
return true;
}
private boolean hasWon(String[] board, char player) {
int size = board.length;
// search the winner horizontally
for (int i = 0; i < size; i++) {
int count = 0;
for (int j = 0; j < size && board[i].charAt(j) == player; j++) {
count++;
}
if (count == size) {
return true;
}
}
// search the winner vertically
for (int j = 0; j < size; j++) {
int count = 0;
for (int i = 0; i < size && board[i].charAt(j) == player; i++) {
count++;
}
if (count == size) {
return true;
}
}
// search the winner diagonally, from left top to right bottom corner
int count = 0;
for (int i = 0; i < size && board[i].charAt(i) == player; i++) {
count++;
}
if (count == size) {
return true;
}
// search the winner diagonally, from left bottom to right top corner
for (int i = 0; i < size && board[size-i-1].charAt(i) == player; i++) {
count++;
}
if (count == size) {
return true;
}
return false;
}
private int count(String[] board, char player) {
int count = 0;
for (String row : board) {
for (char c : row.toCharArray()) {
if (c != player) {
continue;
}
count++;
}
}
return count;
}
}