48 Rotate Image

Leetcode

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).

You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.

Example 1:

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]

Example 2:

Input: matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
Output: [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]

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四个对应的格子交换。从最外层到最内层。

public void rotate(int[][] matrix) {
    int len=matrix.length;
    for(int i=0;i<len/2;++i){
        int i1,i2,i3,i4,j1,j2,j3,j4;
        i1=i; j1=i;
        i2=i1; j2=len-1-j1;
        i3=len-1-i1; j3=j2;
        i4=i3; j4=j1;
        while(j1!=j2){
            int t=matrix[i1][j1];
            matrix[i1][j1]=matrix[i4][j4];
            matrix[i4][j4]=matrix[i3][j3];
            matrix[i3][j3]=matrix[i2][j2];
            matrix[i2][j2]=t;
            j1++; i2++; j3--; i4--;
        }
    }
}