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Author: prateek27

11 Backtracking/n_queen_all_ways.cpp

@@ -0,0 +1,82 @@
+
+#include <iostream>
+using namespace std;
+
+bool canPlace(int board[][20],int n,int x,int y){
+
+    //Column
+    for(int k=0;k<x;k++){
+        if(board[k][y]==1){
+            return false;
+        }
+    }
+    //Left Diag 
+    int i = x;
+    int j = y;
+    while(i>=0 and j>=0){
+        if(board[i][j]==1){
+            return false;
+        }
+        i--; j--;
+    }
+
+    //Right Diag
+    i = x;
+    j = y;
+    while(i>=0 and j<n){
+        if(board[i][j]==1){
+            return false;
+        }
+        i--; j++;
+    }
+    return true;
+}
+
+void printBoard(int n,int board[][20]){
+
+    for(int i=0;i<n;i++){
+        for(int j=0;j<n;j++){
+            cout<< board[i][j]<<" ";
+        }
+        cout<<endl;
+    }
+    cout <<endl;
+}
+//Return type 
+int solveNQueen(int n,int board[][20],int i){
+    //base case 
+    if(i==n){
+        //Print the board
+        printBoard(n,board);
+        return 1;
+    }
+
+
+    // rec case
+    // try to place a queen in every row
+    int ways = 0;
+    for(int j=0;j<n;j++){
+        //whether the current i,j is safe or not 
+        if(canPlace(board,n,i,j)){
+            board[i][j] = 1;
+            ways += solveNQueen(n,board,i+1);
+            //backtrack
+            board[i][j] = 0;
+        }
+    }
+    return ways;
+}
+
+int main() {
+	// your code goes here
+    int board[20][20] = {0};
+    int n;
+    cin>>n;
+
+    solveNQueen(n,board,0);
+
+
+
+	return 0;
+}
+

11 Backtracking/n_queen_print_one.cpp

@@ -0,0 +1,84 @@
+
+#include <iostream>
+using namespace std;
+
+bool canPlace(int board[][20],int n,int x,int y){
+
+    //Column
+    for(int k=0;k<x;k++){
+        if(board[k][y]==1){
+            return false;
+        }
+    }
+    //Left Diag 
+    int i = x;
+    int j = y;
+    while(i>=0 and j>=0){
+        if(board[i][j]==1){
+            return false;
+        }
+        i--; j--;
+    }
+
+    //Right Diag
+    i = x;
+    j = y;
+    while(i>=0 and j<n){
+        if(board[i][j]==1){
+            return false;
+        }
+        i--; j++;
+    }
+    return true;
+}
+
+void printBoard(int n,int board[][20]){
+
+    for(int i=0;i<n;i++){
+        for(int j=0;j<n;j++){
+            cout<< board[i][j]<<" ";
+        }
+        cout<<endl;
+    }
+    cout <<endl;
+}
+
+bool solveNQueen(int n,int board[][20],int i){
+    //base case 
+    if(i==n){
+        //Print the board
+        printBoard(n,board);
+        return true;
+    }
+
+
+    // rec case
+    // try to place a queen in every row
+    for(int j=0;j<n;j++){
+        //whether the current i,j is safe or not 
+        if(canPlace(board,n,i,j)){
+            board[i][j] = 1;
+            bool success = solveNQueen(n,board,i+1);
+            if(success){
+                return true;
+            }
+            //backtrack
+            board[i][j] = 0;
+        }
+    }
+    return false;
+}
+
+int main() {
+	// your code goes here
+    int board[20][20] = {0};
+    int n;
+    cin>>n;
+
+    solveNQueen(n,board,0);
+
+
+
+	return 0;
+}
+