NC14题解 | #按之字形顺序打印二叉树#

• 解法1：队列+flag标志位
• 解法2：双堆栈

解法1：队列+flag标志位

import java.util.*;

/*
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) {
        boolean flag = true;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(pRoot);
        ArrayList<ArrayList<Integer>> res =new ArrayList();
        if(pRoot == null){
            return res;
        }
        while(!queue.isEmpty()){
            int size = queue.size();
            ArrayList<Integer> list = new ArrayList();
            for(int i = 0; i < size; i++){
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left!=null){
                    queue.add(node.left);
                }
                if(node.right!=null){
                    queue.add(node.right);
                }
            }
            if(!flag){
                Collections.reverse(list);
            }
            flag = !flag;
            res.add(list);
        }
        return res;
    }

解法2：双堆栈

public class Solution {
    public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> res = new ArrayList();
        if(pRoot == null){
            return res;
        }
        Stack<TreeNode> lStack = new Stack();
        Stack<TreeNode> rStack = new Stack();
        lStack.push(pRoot);
        while(!lStack.isEmpty() || !rStack.isEmpty()){
            if(!lStack.isEmpty()){
                ArrayList<Integer> list1 = new ArrayList();
                int size1 = lStack.size();
                for(int i = 0; i < size1; i++){
                    TreeNode node = lStack.pop();
                    list1.add(node.val);
                    if(node.left!=null){
                        rStack.push(node.left);
                    }
                    if(node.right!=null){
                        rStack.push(node.right);
                    }
                }
                res.add(list1);
            }
            if(!rStack.isEmpty()){
                ArrayList<Integer> list2 = new ArrayList();
                int size2 = rStack.size();
                for(int i = 0; i < size2; i++){
                    TreeNode node = rStack.pop();
                    list2.add(node.val);
                    if(node.right!=null){
                        lStack.push(node.right);
                    }
                    if(node.left!=null){
                        lStack.push(node.left);
                    }
                }
                res.add(list2);
            }
        }
        return res;
    }
}