Add 51_N-Queens.java

Author: Sean Prashad

Backtracking/51_N-Queens.java

@@ -0,0 +1,56 @@
+class Solution {
+ public List<List<String>> solveNQueens(int n) {
+ if (n == 0) {
+ return Collections.emptyList();
+ }
+
+ char[][] board = new char[n][n];
+ for (int i = 0; i < n; i++) {
+ for (int j = 0; j < n; j++) {
+ board[i][j] = '.';
+ }
+ }
+
+ List<List<String>> result = new ArrayList<>();
+ dfs(board, 0, result);
+ return result;
+ }
+
+ private List<String> constructBoard(char[][] board) {
+ List<String> result = new ArrayList<>();
+
+ for (int i = 0; i < board.length; i++) {
+ String row = new String(board[i]);
+ result.add(row);
+ }
+
+ return result;
+ }
+
+ private boolean isValid(char[][] board, int row, int col) {
+ for (int i = 0; i < board.length; i++) {
+ for (int j = 0; j < board[i].length; j++) {
+ if (board[i][j] == 'Q' && (row == i || row + j == col + i || row + col == i + j)) {
+ return false;
+ }
+ }
+ }
+
+ return true;
+ }
+
+ private void dfs(char[][] board, int colIdx, List<List<String>> result) {
+ if (colIdx == board.length) {
+ result.add(constructBoard(board));
+ return;
+ }
+
+ for (int i = 0; i < board.length; i++) {
+ if (isValid(board, i, colIdx)) {
+ board[i][colIdx] = 'Q';
+ dfs(board, colIdx + 1, result);
+ board[i][colIdx] = '.';
+ }
+ }
+ }
+}