71.二叉树的锯齿形层次遍历

描述

给出一棵二叉树，返回其节点值的锯齿形层次遍历（先从左往右，下一层再从右往左，层与层之间交替进行）

样例

给出一棵二叉树 {3,9,20,#,#,15,7}

        3
       / \
      9  20
         /  \
        15   7

返回其锯齿形的层次遍历为：

    [
      [3],
      [20,9],
      [15,7]
    ]

代码

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */


public class Solution {
    /*
     * @param root: A Tree
     * @return: A list of lists of integer include the zigzag level order traversal of its nodes' values.
     */
    public List> zigzagLevelOrder(TreeNode root) {
        List> result = new ArrayList<>();

        if (root == null) {
            return result;
        }

        Stack currLevel = new Stack();
        Stack nextLevel = new Stack();
        // 不需要新建栈
        Stack tmp;
        
        currLevel.push(root);
        // normal代表当前层顺序是不是从左往右
        boolean normalOrder = true;
        // 因为curLevel和nextLevel不断来回交换，第一个while用于判断树是否遍历完最后一层
        while (!currLevel.isEmpty()) {
            // 每一层都要新建一个动态数组存储当前层的值
            ArrayList currLevelResult = new ArrayList();
            
            // 第二个while用于判断当前层结点是否遍历完成
            while (!currLevel.isEmpty()) {
                TreeNode node = currLevel.pop();
                currLevelResult.add(node.val);
                
                // 栈的弹出是反的，当前层正常顺序，下一层先压左后压右，先弹右后弹左
                if (normalOrder) {
                    if (node.left != null) {
                        nextLevel.push(node.left);
                    }
                    if (node.right != null) {
                        nextLevel.push(node.right);
                    }
                } else {
                    if (node.right != null) {
                        nextLevel.push(node.right);
                    }
                    if (node.left != null) {
                        nextLevel.push(node.left);
                    }
                }
            }

            result.add(currLevelResult);
            // 交换两个栈的引用指向，当前栈要pop成空栈，子结点加入到nextLevel
            tmp = currLevel;
            currLevel = nextLevel;
            nextLevel = tmp;
            normalOrder = !normalOrder;
        }

        return result;
    }
}