add: N-Queen Backtrack solution

Java/LeetCodeN-Queens.java

@@ -0,0 +1,67 @@
+/* Question Link: https://leetcode.com/problems/n-queens/
+ * The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other.
+ * Given an integer n, return all distinct solutions to the n-queens puzzle. You may return the answer in any order.
+ * Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space, respectively.
+ */
+
+
+
+
+
+
+class Solution {
+    public List<List<String>> solveNQueens(int n) {
+        List<List<String>> results = new ArrayList<>();
+        if(n ==0) return results;
+        backtrace(results, new ArrayList<String>(), 0, n, new boolean[n]);
+        return results;
+    }
+
+     public static void backtrace(List<List<String>> result, List<String> list, int level, int n, boolean[] used){
+        if(level == n) result.add(new ArrayList<String>(list));
+        else{
+            for(int i = 0; i < n; i++){
+                if(used[i]) continue;
+                if(isValid(list, level, i, n)){
+                    list.add(createQueen(n, i));
+                    used[i] = true;
+                    backtrace(result, list, level + 1, n, used);
+                    used[i] = false;
+                    list.remove(list.size() - 1);
+                }
+            }
+        }
+    }
+    public static boolean isValid(List<String> list, int row, int column, int n){
+        if(row > 0){
+            String cmp = list.get(row - 1);
+            for(int i = 0; i < row; i++)
+                if(list.get(i).charAt(column) == 'Q') return false;
+            int tempRow = row;
+            int tempCol = column;
+            while(--tempRow >= 0 && --tempCol >= 0){
+                if(list.get(tempRow).charAt(tempCol) == 'Q') return false;
+            }
+            tempRow = row;
+            tempCol = column;
+            while(--tempRow >= 0 && ++tempCol <= n-1)
+                if(list.get(tempRow).charAt(tempCol) == 'Q') return false;
+        }
+        return true;
+    }
+    private static String createQueen(int n, int index){
+        StringBuilder sb = new StringBuilder();
+        int i = 0;
+        for(; i < index; i++)
+            sb.append('.');
+        sb.append('Q');
+        for(++i; i < n; i++){
+            sb.append('.');
+        }
+        return sb.toString();
+    }
+
+
+}
+
+/*contributed by: nilesh05apr */