leetcode N-Queens

任然是NP问题，解决此类问题主要思想就是递归，

就是用一个循环去枚举当前所有情况，然后把元素加入，递归，再把元素移除

代码1 提示
Output Limit Exceeded 输出结果中有重复的

class Solution {
public:
void solveNQueensHelper(vector<vector<string> > &result, vector<string> &tempResult, vector<bool> &flagRow, vector<bool> &flagCol, int &num, int n)
{
if(num==n)
{
result.push_back(tempResult);
return ;
}

for(int i = 0; i < n; ++i)
{
if(flagRow[i]==false)
continue;
for(int j = 0; j < n; ++j)
{
if(flagCol[j]==false)
continue;

tempResult[i][j] = 'Q';
flagRow[i] = false;
flagCol[j] = false;
num++;
solveNQueensHelper(result, tempResult, flagRow, flagCol, num, n);
num--;
flagRow[i] = true;
flagCol[j] = true;
tempResult[i][j] = '.';

}
}

return ;
}

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

if(n==0)
return result;

vector<bool> flagRow(n, true);
vector<bool> flagCol(n, true);
vector<string> tempResult;
for(int i = 0; i < n; ++i)
{
string str = "";
for(int j = 0; j < n; ++j)
str += '.';

tempResult.push_back(str);
}

int num = 0;
solveNQueensHelper(result, tempResult, flagRow, flagCol, num, n);

return result;
}
};

Accepted 代码 参考 http://www.cnblogs.com/TenosDoIt/p/3801621.html

class Solution {
public:

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

if(n==0)
return result;

vector<int> state(n,-1);

solveNQueensHelper(result, state, 0);

return result;
}

void solveNQueensHelper(vector<vector<string> > &result, vector<int> &state, int row)
{
int n = state.size();
if(row==n)
{
vector<string> temp(n, string(n, '.'));
for(int i = 0; i < n; ++i)
{
temp[i][state[i]] = 'Q';
}
result.push_back(temp);
return ;
}

for(int col = 0; col < n; ++col)
{
if(isValid(state, row, col))
{
state[row] = col;
solveNQueensHelper(result, state, row+1);
state[row] = -1;

}
}

return ;
}

bool isValid(vector<int> &state, int row, int col)
{
for(int i = 0; i < row; ++i)
if(state[i]==col || abs(row - i) == abs(col - state[i]))
return false;

return true;

}
};