Q1.6 To rotate a image by 90 degress.

Q: Given an image represented by an NxN matrix, where each pixel in theimage is 4 bytes, write a method to rotate the image by 90 degrees.Can you do this in place?

A:

思路一：第一步交换主对角线两侧的对称元素，第二步交换第i行和第n-1-i行。即先交换主对角线对称的元素，在交换关于水平N/2线对称的元素。

思路二： 逐层旋转。 上->右， 右->下， 下->左， 左->上

#include<iostream>
#include<vector>
using namespace std;

void swap(int &a, int &b){
int t = a;
a = b;
b = t;
}

void rotateImage1(int matrix[][4], int n) {
for (int i = 0; i < n; i++) {
for (int j = 0; j < n - 1 - i; j++ ) {
swap(matrix[i][j], matrix[n-j-1][n-i-1]);
}
}
for (int i = 0; i < n/2; i++) {
for (int j = 0; j < n; j++) {
swap(matrix[i][j], matrix[n-1-i][j]);
}
}
return ;
}

void rotateImage2(int matrix[][4], int n) {
for (int layer = 0; layer < n/2; layer++) {
int first = layer;
int last = n - 1 - layer;
for (int i = first; i < last; i++) {
int offset = i - first;
int top = matrix[first][i];
matrix[first][i] = matrix[last-offset][first];
matrix[last-offset][first] = matrix[last][last - offset];
matrix[last][last - offset] = matrix[i][last];
matrix[i][last] = top;
}
}
}

int main() {
int a[4][4] = {
{1,2,3,4},
{5,6,7,8},
{9,10,11,12},
{13,14,15,16}
};
for(int i=0; i<4; ++i){
for(int j=0; j<4; ++j)
cout<<a[i][j]<<" ";
cout<<endl;
}
cout<<endl;
rotateImage2(a, 4);
for(int i=0; i<4; ++i){
for(int j=0; j<4; ++j)
cout<<a[i][j]<<" ";
cout<<endl;
}
return 0;
}