二叉树的层序遍历

流程

二叉树的层序遍历，就是图的宽度优先遍历，用队列实现：

①准备一个队列，根节点入队

②当前队首元素出队cur，打印

③cur有左孩子就左孩子入队，有右孩子就右孩子入队，即先左后右；

④重复② ~ ③，直到队列空为止。

代码实现

C++ 版

/************************************************************************* > File Name: 028.二叉树的层序遍历.cpp > Author: Maureen > Mail: [email protected] > Created Time: 一 6/20 09:02:18 2022 ************************************************************************/

#include <iostream>
#include <queue>
using namespace std;

class TreeNode {
    
public:
    int val;
    TreeNode *lchild;
    TreeNode *rchild;

    TreeNode(int v) : val(v) {
    }
};

void level(TreeNode *root) {
    
    if (root == nullptr) return ;

    cout << "level-order:";
    queue<TreeNode *> que;
    que.push(root);
    TreeNode *cur = nullptr;
    while (!que.empty()) {
    
        cur = que.front(); que.pop();
        cout << cur->val << " ";

        if (cur->lchild != nullptr) que.push(cur->lchild);
        if (cur->rchild != nullptr) que.push(cur->rchild);
    }
    cout << endl;
}

int main() {
    
    TreeNode *root = new TreeNode(1);
    root->lchild = new TreeNode(2);
    root->rchild = new TreeNode(3);
    root->lchild->lchild = new TreeNode(4);
    root->lchild->rchild = new TreeNode(5);
    root->rchild->lchild = new TreeNode(6);
    root->rchild->rchild = new TreeNode(7);

    level(root);
    return 0;
}

Java 版

import java.util.LinkedList;
import java.util.Queue;

public class LevelTraversalBT {
    

	public static class Node {
    
		public int value;
		public Node left;
		public Node right;

		public Node(int v) {
    
			value = v;
		}
	}

	public static void level(Node head) {
    
		if (head == null) {
    
			return;
		}
		Queue<Node> queue = new LinkedList<>();
		queue.add(head);
		while (!queue.isEmpty()) {
    
			Node cur = queue.poll();
			System.out.println(cur.value);
			if (cur.left != null) {
    
				queue.add(cur.left);
			}
			if (cur.right != null) {
    
				queue.add(cur.right);
			}
		}
	}

	public static void main(String[] args) {
    
		Node head = new Node(1);
		head.left = new Node(2);
		head.right = new Node(3);
		head.left.left = new Node(4);
		head.left.right = new Node(5);
		head.right.left = new Node(6);
		head.right.right = new Node(7);

		level(head);
		System.out.println("========");
	}
}