Complete Backtracking-3

NQueens.java

@@ -0,0 +1,56 @@
+import java.util.*;
+
+// Row wise 0 to n
+// O(n * n!) time, O(n) space
+class Solution {
+    public List<List<String>> solveNQueens(int n) {
+        List<List<String>> ans = new ArrayList<>();
+        char[][] board = new char[n][n];
+
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < n; j++) {
+                board[i][j] = '.';
+            }
+        }
+        helper(0, n, board, ans);
+        return ans;
+    }
+
+    private void helper(int row, int n, char[][] board, List<List<String>> ans) {
+        if (row == n) {
+            List<String> path = new ArrayList<>();
+            for (int i = 0; i < n; i++) {
+                path.add(new String(board[i])); // new string for each board
+            }
+            ans.add(new ArrayList<>(path));
+            return;
+        }
+
+        for (int col = 0; col < n; col++) {
+            if (isValid(row, col, n, board)) {
+                board[row][col] = 'Q';
+                helper(n, row+1, board, ans);
+                board[row][col] = '.'; //backtrack
+            }
+        }
+    }
+
+    private boolean isValid(int row, int col, int n, char[][] board) {
+        // check vertical
+        for (int i = 0; i < row; i++) {
+            if (board[i][col] == 'Q') return false;
+        }
+
+        // check left upper
+        for (int i = row, j = col; i >= 0 && j >= 0; i--, j--) {
+            if (board[i][j] == 'Q') return false;
+        }
+
+        // check right upper
+        for (int i = row, j = col; i >= 0 && j < n; i--, j++) {
+            if (board[i][j] == 'Q') return false;
+        }
+
+        return true;
+    }
+}

WordSearch.java

@@ -0,0 +1,48 @@
+// O(n * m * 3^L) time, O(L) space
+class Solution {
+    public boolean exist(char[][] board, String word) {
+        int n = board.length;
+        int m = board[0].length;
+
+        int[][] dirs = new int[][] {
+            {0,1},{0,-1},{1,0},{-1,0}
+        };
+
+        boolean[][] visited = new boolean[n][m];
+
+        for (int i = 0; i < n; i++) {
+            for (int j = 0; j < m; j++) {
+                if (board[i][j] == word.charAt(0)) {
+                    if (dfs(board, i, j, n, m, word, 0, dirs, visited)) {
+                        return true;
+                    }
+                }
+            }
+        }
+        return false;
+    }
+
+    private boolean dfs(char[][] board, int i, int j, int n, int m, String word, int wordIdx, int[][] dirs, boolean[][] visited) {
+        if (board[i][j] != word.charAt(wordIdx)) return false;
+
+        if (wordIdx == word.length() - 1) return true;
+
+        visited[i][j] = true; // mark as visited
+
+        for (int[] dir : dirs) {
+            int r = i + dir[0];
+            int c = j + dir[1];
+
+            if (r >= 0 && c >= 0 && r < n && c < m && !visited[r][c]) {
+                if (dfs(board, r, c, n, m, word, wordIdx+1, dirs, visited)) {
+                    visited[r][c] = false;
+                    return true;
+                }
+            }
+        }
+
+        visited[i][j] = false; // backtrack
+
+        return false;
+    }
+}