Create 0036-valid-sudoku.java

Author: sanajitjana

Medium/0036-valid-sudoku.java

@@ -0,0 +1,94 @@
+// LeetCode: https://leetcode.com/problems/valid-sudoku/
+// ---------------------------------------------------------
+// Problem Statement:
+// Determine if a 9 x 9 Sudoku board is valid. 
+// Only the filled cells need to be validated according to these rules:
+//   1. Each row must contain the digits 1-9 without repetition.
+//   2. Each column must contain the digits 1-9 without repetition.
+//   3. Each of the nine 3 x 3 sub-boxes must contain the digits 1-9 without repetition.
+// Note: A Sudoku board (partially filled) could be valid but is not necessarily solvable.
+//
+// ---------------------------------------------------------
+// Approach:
+// - Use a HashSet to track digits in each row, column, and 3x3 sub-box.
+// - For rows: Iterate row-wise and check duplicates.
+// - For columns: Iterate col-wise and check duplicates.
+// - For 3x3 sub-boxes: Divide board into 9 blocks using (blockRow, blockCol)
+//   and iterate inside each 3x3 block.
+// - If at any point a duplicate is found, return false.
+// - If all checks pass, return true.
+//
+// ---------------------------------------------------------
+// Complexity Analysis:
+// Time Complexity: O(9^2) = O(81) ≈ O(1)
+//   - We scan each cell at most 3 times (row, col, box).
+//   - Constant-time operations in sets.
+//   - Overall complexity is effectively constant since board size is fixed.
+//
+// Space Complexity: O(9) per row/col/box = O(1)
+//   - Extra space comes from the HashSet used in each validation step.
+//   - Board size is fixed, so extra memory usage is constant.
+//
+// ---------------------------------------------------------
+// Notes for Future:
+// - This solution validates *only* the current board state. It does not attempt to solve the Sudoku.
+// - If extending this to a Sudoku solver, backtracking would be required.
+// ---------------------------------------------------------
+
+import java.util.HashSet;
+import java.util.Set;
+
+class Solution {
+    public boolean isValidSudoku(char[][] board) {
+
+        int m = board.length;
+        int n = board[0].length;
+
+        // Check rows
+        for (int i = 0; i < m; i++) {
+            Set<Character> set = new HashSet<>();
+            for (int j = 0; j < n; j++) {
+                char ch = board[i][j];
+                if (ch != '.') {
+                    if (set.contains(ch))
+                        return false;
+                    else
+                        set.add(ch);
+                }
+            }
+        }
+
+        // Check cols
+        for (int i = 0; i < m; i++) {
+            Set<Character> set = new HashSet<>();
+            for (int k = 0; k < m; k++) {
+                char ch = board[k][i];
+                if (ch != '.') {
+                    if (set.contains(ch))
+                        return false;
+                    else
+                        set.add(ch);
+                }
+            }
+        }
+
+        // Check 3x3 sub-boxes
+        for (int blockRow = 0; blockRow < 3; blockRow++) {
+            for (int blockCol = 0; blockCol < 3; blockCol++) {
+                Set<Character> set = new HashSet<>();
+                for (int i = 0; i < 3; i++) {
+                    for (int j = 0; j < 3; j++) {
+                        char ch = board[blockRow * 3 + i][blockCol * 3 + j];
+                        if (ch != '.') {
+                            if (set.contains(ch))
+                                return false;
+                            set.add(ch);
+                        }
+                    }
+                }
+            }
+        }
+
+        return true;
+    }
+}