8queens

interview_prep/algorithm/java/ide_handicapped/nqueens/Solution.java

@@ -0,0 +1,100 @@
+import java.util.AbstractList;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+class Solution {
+    List<List<String>> result;
+    public List<List<String>> solveNQueens(int n) {
+        return new AbstractList<List<String>>(){
+            @Override
+            public int size(){
+                init();
+                return result.size();
+            }
+
+            @Override
+            public List<String> get(int index){
+                init();
+                return result.get(index);
+            }
+
+            private void init(){
+                if(result != null) return;
+
+                int[][] grid = new int[n][n];
+                result = new ArrayList<>();
+                for(int[] row : grid){
+                    Arrays.fill(row, 0);
+                }
+                getQueenCombinations(grid, n, 0, 0);
+            }
+        };
+    }
+
+    private void getQueenCombinations(
+        int[][] grid,
+        int n,
+        int row,
+        int col
+    ){
+        if(row >= n){
+            result.add(build(grid, n));
+            return;
+        }
+
+        for(int i = 0; i < n; i++){
+            if(isValidCombination(grid, n, row, i)){
+                grid[row][i] = 1;
+                getQueenCombinations(grid, n, row + 1, col);
+                grid[row][i] = 0;
+            }
+        }
+    }
+
+    private boolean isValidCombination(
+        int[][] grid,
+        int n,
+        int row,
+        int col
+    ){
+        for(int i = 0; i < n; i++){
+            for(int j = 0; j < n; j++){
+                if(grid[i][j] == 1 && (row + col == i + j || row + j == col + i || j == col)){
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+
+    private List<String> build(int[][] grid, int n){
+        final List<String> solution = new ArrayList<>();
+        for(int i = 0; i < n; i++){
+            StringBuilder sb = new StringBuilder();
+            for(int j = 0; j < n; j++){
+                if(grid[i][j] == 0){
+                    sb.append('.');
+                }
+                else{
+                    sb.append('Q');
+                }
+            }
+            solution.add(sb.toString());
+        }
+        return solution;
+    }
+
+    public static void main(String[] args) {
+        //https://leetcode.com/problems/n-queens/
+        int boardSize = 8;
+        final Solution solution = new Solution();
+        final List<List<String>> solutions = solution.solveNQueens(boardSize);
+
+        for (List<String> solutionBoard : solutions) {
+            for (String row : solutionBoard) {
+                System.out.println(row);
+            }
+            System.out.println();
+        }
+    }
+}