剑指Offer_59

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题目

请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

解题思路

解法一：

简洁版本，在数据大的时候效率并不高，因为偶数层的AarrayList总是操作Collections.revers(A)

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import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Collections;
/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}
*/
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
		ArrayList<ArrayList<Integer>> AA = new ArrayList<>();
        if(pRoot == null){
            return AA;
        }
        LinkedList<TreeNode> nodeList = new LinkedList<>();
        nodeList.add(pRoot);
        boolean isEven = false;
        while(!nodeList.isEmpty()){
            ArrayList<Integer> A = new ArrayList<>();
           
           //这里的size要保存，nodeList的size是动态的 
            int size = nodeList.size();
            for(int i = 0; i < size; ++i){
                TreeNode node = nodeList.removeFirst();
                A. add(node.val);
                if(node.left != null){
                    nodeList.add(node.left);
                }
                if(node.right != null){
                    nodeList.add(node.right);
                }
            }
            //偶数层的元素逆序
            if(isEven){
                Collections.reverse(A);
            }
            isEven = !isEven;
            AA.add(A);
        }
        return AA;
    }
}

解法二：

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import java.util.ArrayList;
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 {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
		ArrayList<ArrayList<Integer>> AA = new ArrayList<>();
        if (pRoot == null) {
            return AA;
        }
        LinkedList<TreeNode> oddList = new LinkedList<>();
        LinkedList<TreeNode> evenList = new LinkedList<>();
        oddList.add(pRoot);
        boolean isEven = false;
        while(!oddList.isEmpty() || !evenList.isEmpty()){
            ArrayList<Integer> A = new ArrayList<>();
            if(isEven){
                while (!evenList.isEmpty()){
                    TreeNode node = evenList.removeFirst();
                    A.add(node.val);
                    if(node.right != null){
                        oddList.addFirst(node.right);
                    }
                    if(node.left != null){
                        oddList.addFirst(node.left);
                    }
                }
            }
            else{
                while(!oddList.isEmpty()){
                    TreeNode node = oddList.removeFirst();
                    A.add(node.val);
                    if(node.left != null){
                        evenList.addFirst(node.left);
                    }
                    if(node.right != null){
                        evenList.addFirst(node.right);
                    }
                }
            }
            isEven = !isEven;
            AA.add(A);
        }
        return AA;
    }
}