Leetcode: N-Queens

The n-queens puzzle is the problem of placing n queens on an n×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[ [".Q..",  // Solution 1  "...Q",  "Q...",  "..Q."], ["..Q.",  // Solution 2  "Q...",  "...Q",  ".Q.."]]

搞到很久，一个变量写错了，总是报Memory Limit Exceeded，郁闷。

class Solution {public:    vector<vector<string> > solveNQueens(int n) {        solveNQueensUtil(0, n);                return result;    }        void solveNQueensUtil(int step, int n) {        if (step == n) {            print(n);            return;        }                for (int i = 0; i < n; ++i) {            cols.push_back(i);            if (canPutQueen(step)) {                solveNQueensUtil(step + 1, n);            }            cols.pop_back();        }    }        bool canPutQueen(int step) {        for (int i = 0; i < step; ++i) {            if (cols[step] == cols[i] || abs(step - i) == abs(cols[step] - cols[i])) {                return false;            }        }                return true;    }        void print(int n) {        vector<string> solve;        for (int i = 0; i < n; ++i) {            string line;            for (int j = 0; j < n; ++j) {                line.push_back(cols[i] == j ? 'Q' : '.');            }            solve.push_back(line);        }        result.push_back(solve);    }    private:    vector<int> cols;    vector<vector<string> > result;};