二叉树层序遍历

原题

　　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},

3
   / \
  9  20
    /  \
   15   7

 

　　return its level order traversal as:

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

 

 

题目大意

　　给定一个二叉树，输出它的每一层的结点。

解题思路

　　使用两队列，一个保存当前处理的层，一个保存下一次要处理的层。只到每一层都处理完。

代码实现

树结点类

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

 

算法实现类

import java.util.ArrayList;
import java.util.Deque;
import java.util.LinkedList;
import java.util.List;

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {

        List<List<Integer>> result = new ArrayList<>();

        if (root == null) {
            return result;
        }

        Deque<TreeNode> cur = new LinkedList<>();
        Deque<TreeNode> sub = new LinkedList<>();
        Deque<TreeNode> exc;

        TreeNode node;
        cur.addLast(root);

        while (!cur.isEmpty()) {
            List<Integer> layout = new LinkedList<>();
            while (!cur.isEmpty()) {
                node = cur.removeFirst();
                layout.add(node.val);

                if (node.left != null) {
                    sub.addLast(node.left);
                }

                if(node.right != null) {
                    sub.addLast(node.right);
                }
            }

            exc = cur;
            cur = sub;
            sub = exc;

            result.add(layout);
        }

        return result;
    }
}