题解 | #顺时针打印矩阵#

import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> printMatrix(int [][] matrix) {
       ArrayList<Integer> ret = new ArrayList<Integer>();
        int r1 = 0, r2 = matrix.length - 1, c1 = 0, c2 = matrix[0].length - 1;
        while (r1 <= r2 && c1 <= c2) {
        // 左--> 右
        for (int i = c1; i <= c2; i++){
            ret.add(matrix[r1][i]);
        }
        // 上-->下
        for (int i = r1 + 1; i <= r2; i++){
            ret.add(matrix[i][c2]);
        }
        if (r1 != r2){
            // 右-->左
            for (int i = c2 - 1; i >= c1; i--){
                ret.add(matrix[r2][i]);
            }
        }
        if (c1 != c2){
            // 下-->上
            for (int i = r2 - 1; i > r1; i--){
                ret.add(matrix[i][c1]);
            }
        }
            r1++; r2--; c1++; c2--;
        }
        return ret;
    }
}