Merge pull request codemistic#613 from nilesh05apr/nilesh

add: N-Queen Backtrack solution

Author: gantavyamalviya

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 */