描述

給出一棵二叉樹(shù)，返回其節(jié)點(diǎn)值的鋸齒形層次遍歷（先從左往右，下一層再?gòu)挠彝螅瑢优c層之間交替進(jìn)行）

樣例

給出一棵二叉樹(shù) {3,9,20,#,#,15,7}

        3
       / \
      9  20
         /  \
        15   7

返回其鋸齒形的層次遍歷為：

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

代碼

/**
 * 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<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();

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

        Stack<TreeNode> currLevel = new Stack<TreeNode>();
        Stack<TreeNode> nextLevel = new Stack<TreeNode>();
        // 不需要新建棧
        Stack<TreeNode> tmp;
        
        currLevel.push(root);
        // normal代表當(dāng)前層順序是不是從左往右
        boolean normalOrder = true;
        // 因?yàn)閏urLevel和nextLevel不斷來(lái)回交換，第一個(gè)while用于判斷樹(shù)是否遍歷完最后一層
        while (!currLevel.isEmpty()) {
            // 每一層都要新建一個(gè)動(dòng)態(tài)數(shù)組存儲(chǔ)當(dāng)前層的值
            ArrayList<Integer> currLevelResult = new ArrayList<Integer>();
            
            // 第二個(gè)while用于判斷當(dāng)前層結(jié)點(diǎn)是否遍歷完成
            while (!currLevel.isEmpty()) {
                TreeNode node = currLevel.pop();
                currLevelResult.add(node.val);
                
                // 棧的彈出是反的，當(dāng)前層正常順序，下一層先壓左后壓右，先彈右后彈左
                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);
            // 交換兩個(gè)棧的引用指向，當(dāng)前棧要pop成空棧，子結(jié)點(diǎn)加入到nextLevel
            tmp = currLevel;
            currLevel = nextLevel;
            nextLevel = tmp;
            normalOrder = !normalOrder;
        }

        return result;
    }
}