36. Valid Sudoku

https://leetcode.com/problems/valid-sudoku/

題解

class Solution {public boolean isValidSudoku(char[][] board) {boolean[][] rows = new boolean[9][9];boolean[][] cols = new boolean[9][9];boolean[][] grids = new boolean[9][9];for (int i = 0; i < 9; i++) {for (int j = 0; j < 9; j++) {if (board[i][j] == '.') continue;int n = board[i][j] - '1';int g = 3 * (i / 3) + (j / 3);if (rows[i][n] || cols[j][n] || grids[g][n]) return false;rows[i][n] = true;cols[j][n] = true;grids[g][n] = true;}}return true;} }

37. Sudoku Solver

https://leetcode.com/problems/sudoku-solver/

題解

思路是：暴力 DFS 無任何記憶，每走到下一個空閑格子，遍歷放進 1 ~ 9 看是否可行。如果 1 ~ 9 都不行，回溯返回上一級。

class Solution {public void solveSudoku(char[][] board) {boolean[][] rows = new boolean[9][9];boolean[][] cols = new boolean[9][9];boolean[][] grids = new boolean[9][9];int filled = 0;for (int i = 0; i < 9; i++) {for (int j = 0; j < 9; j++) {if (board[i][j] != '.') {int n = board[i][j] - '1';int g = 3 * (i / 3) + (j / 3);rows[i][n] = true;cols[j][n] = true;grids[g][n] = true;filled++;}}}for (int i = 0; i < 9; i++) {for (int j = 0; j < 9; j++) {if (board[i][j] == '.') {for (int num = 1; num <= 9; num++) {if (tryFill(board, i, j, num, rows, cols, grids, filled)) return;}}}}}public boolean tryFill(char[][] board, int i, int j, int value, boolean[][] rows, boolean[][] cols, boolean[][] grids, int filled) {int n = value - 1;int g = 3 * (i / 3) + (j / 3); // 九宮格的indexif (rows[i][n] || cols[j][n] || grids[g][n]) { // 當前數字沖突return false;} else { // 當前數字有效board[i][j] = (char) ('0' + value);rows[i][n] = true;cols[j][n] = true;grids[g][n] = true;if (++filled == 9 * 9) return true;// 填充下一個位置for (int ii = 0; ii < 9; ii++) {for (int jj = 0; jj < 9; jj++) {if (board[ii][jj] == '.') { // 首個下一個位置for (int v = 1; v <= 9; v++) {if (tryFill(board, ii, jj, v, rows, cols, grids, filled)) return true;}// value=1~9都不能填充下一個位置，直接返回board[i][j] = '.';rows[i][n] = false;cols[j][n] = false;grids[g][n] = false;return false;}}}}return false;} }

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