二叉樹層序遍歷

原題

　　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]
]

 

 

題目大意

　　給定一個二叉樹，輸出它的每一層的結點。

解題思路

　　使用兩隊列，一個保存當前處理的層，一個保存下一次要處理的層。只到每一層都處理完。

代碼實現

樹結點類

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

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

 

算法實現類

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;
    }
}