Generate Parentheses,Rotate Image

Generate Parentheses

 

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

"((()))", "(()())", "(())()", "()(())", "()()()"

和编程之美，找钱问题，还有出栈入栈很像，记得一起总结，所有的个数是卡特兰特数，其次是注意转折点是满足left < right。

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> ans;
        if(n <= 0)
            return ans;
        generate(n, n, "", ans);
        return ans;
    }

    void generate(int leftNum,int rightNum,string s,vector<string> &result)
    {
        if(leftNum==0&&rightNum==0)
        {
            result.push_back(s);
        }
        if(leftNum>0)
        {
            generate(leftNum-1,rightNum,s+'(',result);//string不是引用，所以在递归中是复制传递，返回调用处还是原来的值，因为使用s+'('
        }
        if(rightNum>0&&leftNum<rightNum)
        {
            generate(leftNum,rightNum-1,s+')',result);
        }
    }
};

Rotate Image

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

 Follow up:
Could you do this in-place?

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) 
{
	int n = matrix.size();
	for(int i = 0; i < n/2; ++ i)//先以中线上下翻转
		for(int j = 0; j < n; ++ j)
			swap(matrix[i][j], matrix[n-1-i][j]);

	for(int i = 0; i < n; ++ i)//再以主对角线翻转
		for(int j = i+1; j < n; ++ j)
			swap(matrix[i][j],matrix[j][i]);
}
};