vault backup: 2022-06-29 09:05:26

CS notes/pages/Leetcode Valid-Sudoku.md

@@ -77,7 +77,52 @@ This should be a follow-up of [[Leetcode Reshape-The-Matrix]], I tried to implem
 
 Besides overthinking, I also spent a lot of time learning how to use sets and init multi-dimensional vectors.
 
+**Take TWO:**
+Use a hash table to store whether an element is present:
+```cpp
+int usedRow[9][10] = {};
+int usedCol[9][10] = {};
+int usedGrid[9][10] = {};
+```
 ### Solution
+
+My new solution using hash map
+```cpp
+class Solution {
+public:
+  bool isValidSudoku(vector<vector<char>> &board) {
+    // from 0 to 9
+    int usedRow[9][10] = {};
+    int usedCol[9][10] = {};
+    int usedGrid[9][10] = {};
+    int grid;
+    int boardVal;
+
+    for (int i = 0; i < 9; i++) {
+      for (int j = 0; j < 9; j++) {
+        grid = i / 3 * 3 + j / 3;
+
+        boardVal = board[i][j];
+        if (boardVal == '.') {
+          continue;
+        }
+        boardVal = boardVal - '0';
+
+        if (usedRow[i][boardVal] || usedCol[j][boardVal] ||
+            usedGrid[grid][boardVal]) {
+          return false;
+        }
+        usedRow[i][boardVal]++;
+        usedCol[j][boardVal]++;
+        usedGrid[grid][boardVal]++;
+      }
+    }
+
+    return true;
+  }
+};
+```
+
 Bad solution :(
 ```cpp
 class Solution {
@@ -186,25 +231,3 @@ public:
 };
 
 ```
-
-Others' using array.
-```cpp
-class Solution {
-public:
-  bool isValidSudoku(vector<vector<char>> &board) {
-    int used1[9][9] = {0}, used2[9][9] = {0}, used3[9][9] = {0};
-
-    for (int i = 0; i < board.size(); ++i)
-      for (int j = 0; j < board[i].size(); ++j)
-        if (board[i][j] != '.') {
-          int num = board[i][j] - '0' - 1, k = i / 3 * 3 + j / 3;
-          if (used1[i][num] || used2[j][num] || used3[k][num])
-            return false;
-          used1[i][num] = used2[j][num] = used3[k][num] = 1;
-        }
-
-    return true;
-  }
-};
-
-```