p102

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LeetCode p102 Binary Tree Level Order Traversal 题解

1.题目：

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

For example:
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]
]

题意：

根据树返回一个列表。

2.解题思路:

见代码 手写一遍过（两遍）开心23333

3.代码

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ansList = new ArrayList<List<Integer>>();
		dp(root, ansList, 0);
		return ansList;
	}

	public void dp(TreeNode root, List<List<Integer>> ans, int deep) {
		if (root == null)
			return;
		if (ans.size() < deep + 1) {
			List<Integer> list = new ArrayList<Integer>();
			ans.add(list);
		}
		List<Integer> aList = ans.get(deep);
		aList.add(root.val);
		dp(root.left, ans, deep + 1);
		dp(root.right, ans, deep + 1);
	}
}

4.一些总结：