spiral-matrix.java

import java.util.*;

class Solution {
    public List<Integer> spiralOrder(int[][] m) {
        List<Integer> res = new ArrayList<>();
        if (m == null || m.length == 0) return res;

        int top = 0, bot = m.length - 1, left = 0, right = m[0].length - 1;

        while (top <= bot && left <= right) {
            for (int c = left; c <= right; c++) res.add(m[top][c]);
            top++;
            for (int r = top; r <= bot; r++) res.add(m[r][right]);
            right--;
            if (top <= bot) {
                for (int c = right; c >= left; c--) res.add(m[bot][c]);
                bot--;
            }
            if (left <= right) {
                for (int r = bot; r >= top; r--) res.add(m[r][left]);
                left++;
            }
        }
        return res;
    }
}