Rotate Image

主要是boundary的条件

public class Solution {
    public void rotate(int[][] matrix) {
        if(matrix==null || matrix.length==0 || matrix[0].length==0) return;
        int len = matrix.length-1;
        for(int i=0; i<=len;i++){
            for(int j=i; j<len-i;j++){ // 注意这里j exclusive to boundary len-i
                int tmp = matrix[i][j];
                matrix[i][j] = matrix[len-j][i];
                matrix[len-j][i] = matrix[len-i][len-j];
                matrix[len-i][len-j] = matrix[j][len-i];
                matrix[j][len-i] = tmp;
            }
        }
    }
}