二叉树系列之zigzag打印二叉树

数据结构二叉树
tree系列, 数据结构, 高频面试题总结

题目详情

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},

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5
  3
 / \
9  20
  /  \
 15   7

返回其锯齿形的层次遍历为：

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2
3
4
5
[
  [3],
  [20,9],
  [15,7]
]

思路：

  • 广度优先遍历
  • 设置flag，看是正序还是逆序，决定如何加入list
    • 若是正序：list.add(node.val)
    • 若是逆序：list.add(0, node.val)
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/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<ArrayList<Integer>> zigzagLevelOrder(TreeNode root) {
        //广度优先遍历
        //设个flag代表正序还是逆序
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (root == null)
            return result;
        boolean flag = true;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            ArrayList<Integer> list = new ArrayList<Integer>();
            for (int  i = 0; i < size; i++) {
                TreeNode treeNode = queue.poll();
                
                if (flag) {
                    list.add(treeNode.val);
                }
                else {
                    list.add(0, treeNode.val);
                }
                
                if (treeNode.left != null) {
                    queue.offer(treeNode.left);
                }
                if (treeNode.right != null) {
                    queue.offer(treeNode.right);
                }
               
            }
            flag = !flag;
            result.add(list);
        }
        return result;
    }
}

思路2

  • 利用堆栈来做

直接上代码：

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/**
 * 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) {
        // write your code here
      
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        
        if (root == null) {
            return result;
        }
        
        Stack<TreeNode> currLevel = new Stack<TreeNode>();
        Stack<TreeNode> nextLevel = new Stack<TreeNode>();
        Stack<TreeNode> tmp;
        
        currLevel.push(root);
        boolean normalOrder = true;
        
        while (!currLevel.isEmpty()) {
            
            ArrayList<Integer> currLevelResult = new ArrayList<Integer>();
            
            while (!currLevel.isEmpty()) {
                TreeNode node = currLevel.pop();
                currLevelResult.add(node.val);
                
                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);
            tmp = currLevel;
            currLevel = nextLevel;
            nextLevel = tmp;
            normalOrder = !normalOrder;
        }
        
        return result;
    }
}

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