leetcode JAVA Binary Tree Level Order Traversal 难度系数3 3.36

原创于 2014-02-05 11:25:07 发布 · 1.2k 阅读

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本文介绍了一种解决二叉树层次遍历问题的方法，并提供了一个具体的示例代码实现。该算法采用递归方式，从根节点开始，按层级顺序返回各节点的值。

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Question:

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,#,#,15,7},

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

confused what "{1,#,2,3}" means?

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        traverse(root,1,results);
        return results;
    }

	private void traverse(TreeNode root, int level,
			ArrayList<ArrayList<Integer>> results) {
		if(root==null) return;
		if(level>results.size())
			results.add(new ArrayList<Integer>());
		results.get(level-1).add(root.val);
		traverse(root.left, level+1, results);
		traverse(root.right, level+1, results);
	}
}