Paste_Image.png

My code:

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        if (root == null)
            return result;

        Stack<TreeNode> s = new Stack<TreeNode>();
        Stack<TreeNode> sSon = new Stack<TreeNode>();
        s.push(root);
        ArrayList<Integer> sample = new ArrayList<Integer>();
        int count = 2;
        while(!s.isEmpty()) {
            TreeNode temp = s.pop();
            sample.add(temp.val);
            if (s.isEmpty()) {  
                if (count % 2 == 0) {           
                    if (temp.left != null)
                        sSon.push(temp.left);
                    if (temp.right != null)
                        sSon.push(temp.right);
                }
                else {
                    if (temp.right != null)
                        sSon.push(temp.right);
                    if (temp.left != null)
                        sSon.push(temp.left);
                }
                
                result.add(sample);
                sample = new ArrayList<Integer>();
                s = sSon;
                sSon = new Stack<TreeNode>();
                count++;
            }
            else {
                if (count % 2 == 0) {           
                    if (temp.left != null)
                        sSon.push(temp.left);
                    if (temp.right != null)
                        sSon.push(temp.right);
                }
                else {
                    if (temp.right != null)
                        sSon.push(temp.right);
                    if (temp.left != null)
                        sSon.push(temp.left);
                }
            }
        }
        return result;
    }
}

My test result:

Paste_Image.png

這道題目也需要動點腦子，總算還是做出來了。
其實和上面那道題目， Binary Tree Level Order Traversal 很類似，只不過改變了遍歷的順序。于是我決定改用Stack。但是也有些細節(jié)需要注意。
比如， 1,2,3,4,#,#,5
如果簡單的把queue換成stack,會這樣
第一次壓入 1， 彈出。
第二次壓入 3， 2， 彈出，所以訪問順序會是 2， 3. 就錯了。
所以插入順序應該反一反，壓入 2， 3. 然后彈出3， 2.
第三次會壓入5， 4. 彈出 4， 5.
這樣的順序就對了。

**
總結: stack, zigzag level traversal
**

Anyway, Good luck, Richardo!

My code:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        ArrayList<List<Integer>> ret = new ArrayList<List<Integer>>();
        if (root == null)
            return ret;
        int counter = 0;
        Stack<TreeNode> s = new Stack<TreeNode>();
        s.push(root);
        while(!s.isEmpty()) {
            ArrayList<Integer> group = new ArrayList<Integer>();
            int size = s.size();
            Stack<TreeNode> help = new Stack<TreeNode>();
            if (counter % 2 == 0) {
                for (int i = 0; i < size; i++) {
                    TreeNode temp = s.pop();
                    group.add(temp.val);
                    if (temp.left != null)
                        help.push(temp.left);
                    if (temp.right != null)
                        help.push(temp.right);
                }
            }
            else {
                for (int i = 0; i < size; i++) {
                    TreeNode temp = s.pop();
                    group.add(temp.val);
                    if (temp.right != null)
                        help.push(temp.right);
                    if (temp.left != null)
                        help.push(temp.left);
                }
            }
            s = help;
            counter++;
            ret.add(group);
        }
        return ret;
    }
}

想簡單了一點，不能只用一個stack實現(xiàn)，得用兩個。
不難。
有時候做題不能想當然。像這道題目，s彈出元素對其操作時，再次把子結點插入該棧中，造成了問題。一開始盲目想的時候都沒有想到。

Anyway, Good luck, Richardo!

My code:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        if (root == null) {
            return ret;
        }
        
        Stack<TreeNode> curr = new Stack<TreeNode>();
        Stack<TreeNode> next = new Stack<TreeNode>();
        curr.push(root);
        int counter = 0;
        while (!curr.isEmpty()) {
            int size = curr.size();
            List<Integer> list = new ArrayList<Integer>();
            for (int i = 0; i < size; i++) {
                TreeNode node = curr.pop();
                list.add(node.val);
                if (counter % 2 == 0) {
                    if (node.left != null) {
                        next.push(node.left);
                    }
                    if (node.right != null) {
                        next.push(node.right);
                    }
                }
                else {
                    if (node.right != null) {
                        next.push(node.right);
                    }
                    if (node.left != null) {
                        next.push(node.left);
                    }
                }
            }
            ret.add(list);
            counter++;
            curr = next;
            next = new Stack<TreeNode>();
        }
        
        return ret;
    }
}

沒什么好說的，感覺就是體力活，沒難度。

Anyway, Good luck, Richardo! -- 09/06/2016