第59题——按之字形顺序打印二叉树

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。

思路：

利用层次遍历，奇数行进入list1，偶数行先进入栈，当栈大小等于队列之前统计的当前层的个数时，栈所有的依次出栈进入list2。

Java

package nowcoder;

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

public class S59_ZPrint {
    public ArrayList<ArrayList<Integer>> ZPrint(TreeNode root){
        ArrayList<ArrayList<Integer>> list = new ArrayList<ArrayList<Integer>>();
        if (root == null)
            return list;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        int layer = 1;//层数
        int layerCount = 1;//当前层的个数
        int count = 0;//计数
        ArrayList<Integer> list1 = new ArrayList<Integer>();
        ArrayList<Integer> list2 = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()){
            TreeNode out = queue.poll();
            count++;
            if (out.left != null){
                queue.offer(out.left);
            }
            if (out.right != null){
                queue.offer(out.right);
            }
            //奇数层
            if (layer %2 == 1){
                list1.add(out.val);
                if (list1.size() == layerCount){
                    list.add(list1);
                    list1 = new ArrayList<Integer>();
                }
            }
            else {
                //偶数层先进栈
                stack.push(out);
                //若栈中的个数==layerCount，则弹出到list2
                if (stack.size() == layerCount){
                    while (!stack.isEmpty())
                        list2.add(stack.pop().val);
                    list.add(list2);
                    list2 = new ArrayList<Integer>();
                }
            }
            if (count == layerCount){
                layer++;
                layerCount = queue.size();
                count = 0;
            }
        }
        return list;
    }
    public static void main(String[] args){
        S59_ZPrint s59 = new S59_ZPrint();
        PrintTreeLayer p = new PrintTreeLayer();
        Integer[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11};
        TreeNode root = p.arrayToTree(array, 0);
        System.out.println(s59.ZPrint(root));
    }
}

结果：

[[1], [3, 2], [4, 5, 6, 7], [11, 10, 9, 8]]

Python

import PrintTreeLayer
class ZPrint:
    def Print(self, pRoot):
        if not pRoot:
            return []
        nodeStack = [pRoot]
        result = []
        while nodeStack:
            res = []
            nextStack = []
            for i in nodeStack:
                res.append(i.val)
                if i.left:
                    nextStack.append(i.left)
                if i.right:
                    nextStack.append(i.right)
            nodeStack = nextStack
            result.append(res)
        returnResult = []
        for i, v in enumerate(result):
            if i % 2 == 0:
                returnResult.append(v)
            else:
                returnResult.append(v[::-1])
        return returnResult
if __name__ == '__main__':
    test = ZPrint()
    p = PrintTreeLayer.PrintTreeLayer()
    array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
    root = p.arrayToTree(array, 0)
    print(test.Print(root))