feat: solve spiral matrix

Author: GangBean

spiral-matrix/GangBean.java

@@ -0,0 +1,37 @@
+class Solution {
+    /**
+    1. understanding
+    - 3 x 3 = N x M
+    - (1,2,3) -> (6,9) // (8,7) -> (4) // (5)
+    - upper: step M count, N -= 1 // right: step N count, M -= 1 // bottom: step M count, N -= 1 // left: step N count, M -= 1
+    2. complexity:
+    - time: O(N * M)
+    - space: O(1)
+    */
+    public List<Integer> spiralOrder(int[][] matrix) {
+        int r = 0;
+        int c = -1;
+        int dir = 1;
+        int N = matrix.length;
+        int M = matrix[0].length;
+        List<Integer> ret = new ArrayList<>();
+
+        while (0 < N && 0 < M) {
+            for (int i = 0; i < M; i++) {
+                c += dir;
+                ret.add(matrix[r][c]);
+            }
+            N -= 1;
+            for (int i = 0; i < N; i++) {
+                r += dir;
+                ret.add(matrix[r][c]);
+            }
+            M -= 1;
+
+            dir *= -1;
+        }
+
+        return ret;
+    }
+}
+