Solved

problem1.java

@@ -0,0 +1,50 @@
+class Solution {
+    List<List<String>> result;
+    int n;
+    public List<List<String>> solveNQueens(int n) {
+        char[][] board = new char[n][n];
+        for (int i=0; i<n; i++) {
+            Arrays.fill(board[i], '.');
+        }
+        result = new ArrayList<>();
+        this.n = n;
+
+        backtrack(board, new HashSet<>(), new HashSet<>(), new HashSet<>(), 0);
+
+        return result;
+    }
+
+    private void backtrack(char[][] board, Set<Integer> colSet, Set<Integer> negativeDiagSet, Set<Integer> positiveDiagSet, int row) {
+
+        if (row == n) {
+            List<String> list = new ArrayList<>();
+            for (int k=0; k<n; k++) {
+                list.add(String.valueOf(board[k]));
+            }
+            result.add(list);
+            return;
+        }
+
+        for (int col = 0; col<n; col++) {
+            int posDiag = row+col;
+            int negDiag = row-col;
+
+            if (colSet.contains(col) || positiveDiagSet.contains(posDiag) || negativeDiagSet.contains(negDiag)) {
+                continue;
+            }
+
+            colSet.add(col);
+            positiveDiagSet.add(posDiag);
+            negativeDiagSet.add(negDiag);
+            board[row][col] = 'Q';
+
+            backtrack(board, colSet, negativeDiagSet, positiveDiagSet, row+1);
+
+            colSet.remove(col);
+            positiveDiagSet.remove(posDiag);
+            negativeDiagSet.remove(negDiag);
+            board[row][col] = '.';
+
+        }
+    }
+}

problem2.java

@@ -0,0 +1,41 @@
+class Solution {
+    int[][] DIRS = new int[][] {{-1, 0}, {0, 1}, {1, 0}, {0, -1}};
+    int m, n;
+    boolean exists;
+    public boolean exist(char[][] board, String word) {
+        if (word == null || word.length() == 0) return true;
+
+        m = board.length;
+        n = board[0].length;
+        exists = false;
+
+        int[][] visited = new int[m][n];
+        for (int i=0; i<board.length; i++) {
+            for (int j=0; j<board[0].length; j++) {
+                if (word.charAt(0) == board[i][j]) {
+                    visited[i][j] = 1;
+                    dfs(board, word, 1, visited, i, j);
+                    visited[i][j] = 0;
+                }
+            }
+        }
+
+        return exists;
+    }
+
+    public void dfs(char[][] board, String word, int idx, int[][] visited, int i, int j) {
+        if (idx == word.length() || exists) {
+            exists = true;
+            return;
+        }
+        for (int[] dir : DIRS) {
+            int newI = i + dir[0];
+            int newJ = j + dir[1];
+            if (newI>=0 && newJ>=0 && newI<m && newJ<n && visited[newI][newJ] == 0 && word.charAt(idx) == board[newI][newJ]) {
+                visited[newI][newJ] = 1;
+                dfs(board, word, idx + 1, visited, newI, newJ);
+                visited[newI][newJ] = 0;
+            }
+        }
+    }
+}