【Rotate Image】cpp

题目：

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?

代码：

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
            const unsigned int len = matrix.size();
            if ( len < 2 ) return;
            for ( int row = 0; row < len; ++row)
            {
                for (int col = 0; col < len-1-row; ++col)
                {
                    std::swap(matrix[row][col], matrix[len-1-col][len-1-row]);
                }
            }
            for ( int row = 0; row < len/2; ++row)
            {
                for ( int col = 0; col < len; ++col)
                {
                    std::swap(matrix[row][col], matrix[len-1-row][col]);
                }
            }
    }
};

Tips:

1. 算法：先沿着副对角线（右上到左下）swap元素；再沿着水平中轴线swap元素

2. 注意循环遍历时的结束条件，不要做无谓的遍历

=============================================

图像旋转90°的算法技巧：先副对角线对折；再沿着水平中轴对折。代码一次AC。

class Solution {
public:
    void rotate(vector<vector<int>>& matrix) {
            const int N = matrix.size();
            if ( N<2 ) return;
            for ( int i=0; i<N; ++i )
            {
                for ( int j=0; j<N-i; ++j )
                {
                    std::swap(matrix[i][j], matrix[N-1-j][N-1-i]);
                }
            }
            for ( int i=0; i<N/2; ++i)
            {
                for ( int j=0; j<N; ++j)
                {
                    std::swap(matrix[i][j], matrix[N-1-i][j]);
                }
            }
    }
};