題目描述

從上到下按層列印二叉樹，同一層結點從左至右輸出。每一層輸出一行。

AC程式碼

層次遍歷二叉樹用到的資料結構是佇列。

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

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
 

public class Solution {
    Queue<TreeNode> queue = new LinkedList<>();
    ArrayList<ArrayList<Integer>> resList = new ArrayList<ArrayList<Integer>>();
    int curNode = 1;//當前層具有的結點數目
    int nextNode = 0;//下一層具有的結點數目
    ArrayList<ArrayList<Integer> > Print
 
(TreeNode pRoot) {
        if(pRoot == null) return resList;
        queue.offer(pRoot);
        TreeNode tmp = null;
        ArrayList<Integer> curList = new ArrayList<Integer>();
        while(!queue.isEmpty()){
            tmp = queue.poll();
            curList.add(tmp.val);
            curNode--
 
;
            if(tmp.left != null) {
                queue.offer(tmp.left);
                nextNode++;
            }
            if(tmp.right != null) {
                queue.offer(tmp.right);
                nextNode++;
            }
            if(curNode == 0){
                resList.add(curList);
                curList = new ArrayList();
                curNode = nextNode;
                nextNode = 0;
            }
        }
        return resList;
    }
    
}