[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.."]
]

DFS to solve it!
 1 class Solution {
 2 private:
 3     vector<vector<string> > ret;
 4     int a[100];
 5     bool canUse[100];
 6 public:
 7     bool check(int y, int n)
 8     {
 9         for(int i = 0; i < n; i++)
10             if (abs(i - n) == abs(y - a[i]))
11                 return false;
12         return true;
13     }
14     
15     void solve(int dep, int maxDep)
16     {
17         if (dep == maxDep)
18         {
19             vector<string> ans;
20             for(int i = 0; i < maxDep; i++)
21             {
22                 string s;
23                 for(int j = 0; j < a[i]; j++)
24                     s += '.';
25                 s += 'Q';
26                 for(int j = 0; j < maxDep - (a[i] + 1); j++)
27                     s += '.';
28                 ans.push_back(s);
29             }
30             ret.push_back(ans);
31             
32             return;
33         }
34         
35         for(int i = 0; i < maxDep; i++)
36             if (canUse[i] && check(i, dep))
37             {
38                 canUse[i] = false;
39                 a[dep] = i;
40                 solve(dep + 1, maxDep);
41                 canUse[i] = true;             
42             }
43     }
44     
45     vector<vector<string> > solveNQueens(int n) {
46         // Start typing your C/C++ solution below
47         // DO NOT write int main() function
48         ret.clear();
49         memset(canUse, true, sizeof(canUse));
50         solve(0, n);
51         return ret;
52     }
53 };
posted @ 2012-11-13 10:58  chkkch  阅读(1983)  评论(0)  编辑  收藏  举报