N-QueenProblem.cpp

class Queen {
public:

bool valid(int row, int col, vector<string>arr){
    int n = arr.size();

    // check row
    for(int i = 0; i < n; i++){
        if(arr[row][i] != '.'){
            return 0;
        }
    }

    // check column
    for(int i = 0; i < n; i++){
        if(arr[i][col] != '.'){
            return 0;
        }
    }

    // check diagonal (upper left to lower right)
    int i = row, j = col;
    while(i >= 0 && j >= 0){
        if(arr[i][j] != '.'){
            return 0;
        }
        i--;
        j--;
    }

    // check diagonal (lower left to upper right)
    i = row, j = col;
    while(i >= 0 && j < n){
        if(arr[i][j] != '.'){
            return 0;
        }
        i--;
        j++;
    }

    return 1;
}


    void dfs(int index, int count, int n, vector<string> arr, vector<vector<string>> &ans ){
        if(count == n){
            ans.push_back(arr);
            return;
        }

        if(index >= n){
            return;
        }

        for(int i=0; i<n; i++){
            if(valid(index, i, arr)){
                arr[index][i] = 'Q';
                dfs(index + 1, count + 1, n, arr, ans);
                arr[index][i] = '.';
            }
        }
    }


    vector<vector<string>> solveNQueens(int n) {
        vector<string> arr;

        string brr;

        vector<vector<string>> ans;
        for(int i = 0; i < n; i++){
            brr += '.';
        }

        for(int i = 0; i < n; i++ ){
            arr.push_back(brr);
        }

        dfs(0, 0, n, arr, ans);

        return ans;
    }
};