diff --git a/036_valid_sudoku/Makefile b/036_valid_sudoku/Makefile
new file mode 100644
index 0000000..fcc56aa
--- /dev/null
+++ b/036_valid_sudoku/Makefile
@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test valid_sudoku.c
diff --git a/036_valid_sudoku/valid_sudoku.c b/036_valid_sudoku/valid_sudoku.c
new file mode 100644
index 0000000..452fb2e
--- /dev/null
+++ b/036_valid_sudoku/valid_sudoku.c
@@ -0,0 +1,139 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+
+struct stack {
+    int row;
+    int col;
+    int value;
+};
+
+static bool valid(char **board, int row, int col)
+{
+    int i, j, k;
+    char mark[10];
+
+    for (i = 0; i <= row; i++) {
+        memset(mark, 0, 10);
+        /* check row validity */
+        for (j = 0; j < 9; j++) {
+            if (board[i][j] != '.') {
+                int index = board[i][j] - '0';
+                if (mark[index]) {
+                    return false;
+                } else {
+                    mark[index] = 1;
+                }
+            }
+        }
+    }
+
+    /* check column validity */
+    for (i = 0; i <= col; i++) {
+        memset(mark, 0, 10);
+        for (j = 0; j < 9; j++) {
+            if (board[j][i] != '.') {
+                int index = board[j][i] - '0';
+                if (mark[index]) {
+                    return false;
+                } else {
+                    mark[index] = 1;
+                }
+            }
+        }
+    }
+
+    /* check sub-box validity */
+    int count = row / 3 * 3 + col / 3 + 1;
+    for (k = 0; k < count; k++) {
+        int sr = k / 3 * 3;
+        int sc = (k % 3) * 3;
+        memset(mark, 0, 10);
+        for (i = sr; i < sr + 3; i++) {
+            for (j = sc; j < sc + 3; j++) {
+                if (board[i][j] != '.') {
+                    int index = board[i][j] - '0';
+                    if (mark[index]) {
+                        return false;
+                    } else {
+                        mark[index] = 1;
+                    }
+                }
+            }
+        }
+    }
+
+    return true;
+}
+
+bool isValidSudoku(char** board, int boardRowSize, int boardColSize) {
+    if (boardRowSize != 9 || boardColSize != 9) {
+        return false;
+    }
+
+    int i = 0, j = 0, k = 1, num = 0;
+    struct stack stack[81];
+
+    while (i < boardRowSize) {
+        if (board[i][j] == '.') {
+            while (k <= 9) {
+                board[i][j] = k + '0';
+                if (valid(board, i, j)) {
+                    stack[num].row = i;
+                    stack[num].col = j;
+                    stack[num].value = k;
+                    num++;
+                    k = 1;
+                    break;
+                }
+                k++;
+            }
+            if (k == 10) {
+                if (num == 0) {
+                    return false;
+                }
+                board[i][j] = '.';
+                --num;
+                i = stack[num].row;
+                j = stack[num].col;
+                k = stack[num].value + 1;
+                board[i][j] = '.';
+                continue;
+            }
+        }
+        /* next row */
+        if (++j == boardColSize) {
+            j = 0;
+            i++;
+        }
+    }
+
+    return true;
+}
+
+int main(int argc, char **argv)
+{
+    int i, j;
+    char *str = argv[1];
+    char **board = malloc(9 * sizeof(char *));
+    for (i = 0; i < 9; i++) {
+        board[i] = malloc(10);
+        memcpy(board[i], str + i * 9, 9);
+        board[9] = '\0';
+        char *row = board[i];
+        for (j = 0; j < 9; j++) {
+            printf("%c ", row[j]);
+        }
+        printf("\n");
+    }
+    printf("%s\n", isValidSudoku(board, 9, 9) ? "true" : "false");
+    for (i = 0; i < 9; i++) {
+        char *row = board[i];
+        for (j = 0; j < 9; j++) {
+            printf("%c ", row[j]);
+        }
+        printf("\n");
+    }
+    return;
+}
diff --git a/037_sudoku_solver/Makefile b/037_sudoku_solver/Makefile
new file mode 100644
index 0000000..debabd7
--- /dev/null
+++ b/037_sudoku_solver/Makefile
@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test sudoku_solver.c
diff --git a/037_sudoku_solver/sudoku_solver.c b/037_sudoku_solver/sudoku_solver.c
new file mode 100644
index 0000000..30f699f
--- /dev/null
+++ b/037_sudoku_solver/sudoku_solver.c
@@ -0,0 +1,138 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+
+struct stack {
+    int row;
+    int col;
+    int value;
+};
+
+static bool valid(char **board, int row, int col)
+{
+    int i, j, k;
+    char mark[10];
+
+    for (i = 0; i <= row; i++) {
+        memset(mark, 0, 10);
+        /* check row validity */
+        for (j = 0; j < 9; j++) {
+            if (board[i][j] != '.') {
+                int index = board[i][j] - '0';
+                if (mark[index]) {
+                    return false;
+                } else {
+                    mark[index] = 1;
+                }
+            }
+        }
+    }
+
+    /* check column validity */
+    for (i = 0; i <= col; i++) {
+        memset(mark, 0, 10);
+        for (j = 0; j < 9; j++) {
+            if (board[j][i] != '.') {
+                int index = board[j][i] - '0';
+                if (mark[index]) {
+                    return false;
+                } else {
+                    mark[index] = 1;
+                }
+            }
+        }
+    }
+
+    /* check sub-box validity */
+    int count = row / 3 * 3 + col / 3 + 1;
+    for (k = 0; k < count; k++) {
+        int sr = k / 3 * 3;
+        int sc = (k % 3) * 3;
+        memset(mark, 0, 10);
+        for (i = sr; i < sr + 3; i++) {
+            for (j = sc; j < sc + 3; j++) {
+                if (board[i][j] != '.') {
+                    int index = board[i][j] - '0';
+                    if (mark[index]) {
+                        return false;
+                    } else {
+                        mark[index] = 1;
+                    }
+                }
+            }
+        }
+    }
+
+    return true;
+}
+
+static void solveSudoku(char** board, int boardRowSize, int boardColSize)
+{
+    if (boardRowSize != 9 || boardColSize != 9) {
+        return;
+    }
+
+    int i = 0, j = 0, k = 1, num = 0;
+    struct stack stack[81];
+
+    while (i < boardRowSize) {
+        if (board[i][j] == '.') {
+            while (k <= 9) {
+                board[i][j] = k + '0';
+                if (valid(board, i, j)) {
+                    stack[num].row = i;
+                    stack[num].col = j;
+                    stack[num].value = k;
+                    num++;
+                    k = 1;
+                    break;
+                }
+                k++;
+            }
+            if (k == 10) {
+                if (num == 0) {
+                    return;
+                }                
+                board[i][j] = '.';
+                --num;
+                i = stack[num].row;
+                j = stack[num].col;
+                k = stack[num].value + 1;
+                board[i][j] = '.';
+                continue;
+            }
+        }
+        /* next row */
+        if (++j == boardColSize) {
+            j = 0;
+            i++;
+        }
+    }
+}
+
+int main(int argc, char **argv)
+{
+    int i, j;
+    char *str = argv[1];
+    char **board = malloc(9 * sizeof(char *));
+    for (i = 0; i < 9; i++) {
+        board[i] = malloc(10);
+        memcpy(board[i], str + i * 9, 9);
+        board[9] = '\0';
+        char *row = board[i];
+        for (j = 0; j < 9; j++) {
+            printf("%c ", row[j]);
+        }
+        printf("\n");
+    }
+    solveSudoku(board, 9, 9);
+    for (i = 0; i < 9; i++) {
+        char *row = board[i];
+        for (j = 0; j < 9; j++) {
+            printf("%c ", row[j]);
+        }
+        printf("\n");
+    }
+    return;
+}