Completed Backtracking-3

N-queens

@@ -0,0 +1,67 @@
+// time O( n! )
+
+// Space Complexity: O( n^2 ) for the board + O( n ) recursive stack space
+
+
+
+
+class Solution {
+    List<List<String>> result;
+    boolean[][] board;
+    boolean[] colSet;
+    boolean[] mainDiagSet;
+    boolean[] antiDiagSet;
+    public List<List<String>> solveNQueens(int n) {
+        this.result = new ArrayList<>();
+        this.board = new boolean[n][n];
+        this.colSet = new boolean[2*n];
+        this.mainDiagSet = new boolean[2*n];
+        this.antiDiagSet = new boolean[2*n];
+
+        helper(0, n);
+
+        return result;
+    }
+
+    private void helper(int r , int n){
+
+        if(r == n){
+            List<String> list = new ArrayList<>();
+            for(int i=0; i<n; i++){
+                StringBuilder sb = new StringBuilder();
+                for(int j=0; j<n; j++){
+                    if(board[i][j]){
+                        sb.append("Q");
+                    }else{
+                        sb.append(".");
+                    }
+                }
+                list.add(sb.toString());
+            }
+            result.add(list);
+            return;
+        }
+
+        for(int c=0; c<n; c++){
+
+            int md = r+c;
+            int ad = r-c + n;
+
+            if(!colSet[c]&& !mainDiagSet[md] && !antiDiagSet[ad])
+            {
+                colSet[c] = true;
+                mainDiagSet[md] = true;
+                antiDiagSet[ad] = true;
+
+                board[r][c] = true;
+                helper(r+1, n);
+                board[r][c] = false;
+
+                colSet[c] = false;
+                mainDiagSet[md] = false;
+                antiDiagSet[ad] = false;
+
+            }
+        }
+    }
+}

Word search

@@ -0,0 +1,83 @@
+// Time Complexity: O(m * n * 4^L) where L = word.length()
+
+// Space Complexity: O(M * N * L)
+
+
+
+
+import java.util.*;
+
+class Solution {
+    public boolean exist(char[][] board, String word) {
+        if (board == null || board.length == 0 || board[0].length == 0) return false;
+
+        int rows = board.length;
+        int cols = board[0].length;
+        int[][] directions = new int[][]{{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
+
+        for (int i = 0; i < rows; i++) {
+            for (int j = 0; j < cols; j++) {
+                if (board[i][j] == word.charAt(0)) {
+                    if (bfs(board, word, i, j, directions)) return true;
+                }
+            }
+        }
+
+        return false;
+    }
+
+    private boolean bfs(char[][] board, String word, int startI, int startJ, int[][] directions) {
+        int rows = board.length;
+        int cols = board[0].length;
+
+        Queue<State> queue = new LinkedList<>();
+        boolean[][] visited = new boolean[rows][cols];
+        visited[startI][startJ] = true;
+        queue.offer(new State(startI, startJ, 0, visited));
+
+        while (!queue.isEmpty()) {
+            State curr = queue.poll();
+            int i = curr.i, j = curr.j, idx = curr.idx;
+            boolean[][] vis = curr.visited;
+
+            if (idx == word.length() - 1) return true;
+
+            for (int[] dir : directions) {
+                int newI = i + dir[0];
+                int newJ = j + dir[1];
+
+                if (newI >= 0 && newJ >= 0 && newI < rows && newJ < cols && !vis[newI][newJ]) {
+                    if (board[newI][newJ] == word.charAt(idx + 1)) {
+                        boolean[][] newVis = deepCopy(vis);
+                        newVis[newI][newJ] = true;
+                        queue.offer(new State(newI, newJ, idx + 1, newVis));
+                    }
+                }
+            }
+        }
+
+        return false;
+    }
+
+    private boolean[][] deepCopy(boolean[][] original) {
+        int m = original.length;
+        int n = original[0].length;
+        boolean[][] copy = new boolean[m][n];
+        for (int i = 0; i < m; i++) {
+            System.arraycopy(original[i], 0, copy[i], 0, n);
+        }
+        return copy;
+    }
+
+    private static class State {
+        int i, j, idx;
+        boolean[][] visited;
+
+        State(int i, int j, int idx, boolean[][] visited) {
+            this.i = i;
+            this.j = j;
+            this.idx = idx;
+            this.visited = visited;
+        }
+    }
+}