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Leetcode102-binaryTreeLevelOrderTraversal

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Solution Report of LeetCode Acceptted

Description

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

Example

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Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]

Solution

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<List<Integer>> res = new ArrayList<>();
    
    public void traversal(TreeNode node, int level){
        if (res.size() <= level) res.add(new ArrayList<>());
        res.get(level).add(node.val);
        if (node.left != null) traversal(node.left, level + 1);
        if (node.right != null) traversal(node.right, level + 1);
    }
    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) return res;
        traversal(root, 0);
        return res;
    }
}