Add 'Sudoku Solver' solution.

Author: liuyingshan

Sudoku_Solver.cpp

@@ -0,0 +1,45 @@
+/*
+  Write a program to solve a Sudoku puzzle by filling the empty cells.
+  Empty cells are indicated by the character '.'.
+  You may assume that there will be only one unique solution.
+*/
+
+class Solution {
+public:
+
+    bool isValid(vector<vector<char> > &board, int x, int y) {
+        for (int i=0; i<9; i++) {
+            if (i != x && board[i][y] == board[x][y]) return false;
+        }
+        for (int j=0; j<9; j++) {
+            if (j != y && board[x][j] == board[x][y]) return false;
+        }
+        int sx = x / 3 * 3, sy = y / 3 * 3;
+        for (int i=sx; i<sx+3; i++) {
+            for (int j=sy; j<sy+3; j++) {
+                if (i != x && j != y && board[i][j] == board[x][y]) return false;
+            }
+        }
+        return true;
+    }
+    
+    bool solveSudokuDFS(vector<vector<char> > &board, int x, int y) {
+        x = x + y / 9;
+        y = y % 9;
+        if (x == 9) return true;
+
+        if (board[x][y] == '.') {
+            for (char k='1'; k<='9'; k++) {
+                board[x][y] = k;
+                if (isValid(board, x, y) && solveSudokuDFS(board, x, y+1)) return true;
+                board[x][y] = '.';
+            }
+            return false;
+        }
+        return solveSudokuDFS(board, x, y+1);
+    }
+    
+    void solveSudoku(vector<vector<char> > &board) {
+        solveSudokuDFS(board, 0, 0);
+    }
+};