tree travel

Author: tanfanhua

src/com/blankj/medium/_0102/Solution.java

@@ -0,0 +1,44 @@
+package com.blankj.medium._0102;
+
+import com.blankj.structure.TreeNode;
+
+import java.util.*;
+
+public class Solution {
+
+    public static void main(String[] args) {
+        Solution s = new Solution();
+        System.out.println(s.levelOrderTraversal(TreeNode.createTestData("[1,9,20,null,null,15,7]")));
+    }
+
+    public List<List<Integer>> levelOrderTraversal(TreeNode head) {
+        if (head == null) {
+            return Collections.emptyList();
+        }
+        TreeMap<Integer, List<Integer>> levelValuesMap = new TreeMap<>();
+        levelValuesMap.put(0, List.of(head.val));
+        travelOneTree(head.left, 1, levelValuesMap);
+        travelOneTree(head.right, 1, levelValuesMap);
+        List<List<Integer>> res = new ArrayList<>();
+        for (List<Integer> value : levelValuesMap.values()) {
+            res.add(value);
+        }
+        return res;
+    }
+
+    private void travelOneTree(TreeNode node, int level, Map<Integer, List<Integer>> levelValuesMap) {
+        if (node == null) {
+            return;
+        }
+        List<Integer> values = levelValuesMap.get(level);
+        if (values == null) {
+            values = new ArrayList<>();
+            levelValuesMap.put(level, values);
+        }
+        values.add(node.val);
+        travelOneTree(node.left, level + 1, levelValuesMap);
+        travelOneTree(node.right, level + 1, levelValuesMap);
+    }
+
+
+}