LeetCode—103. Binary Tree Zigzag Level Order Traversal

LeetCode—103. Binary Tree Zigzag Level Order Traversal

题目

https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/description/
之字形层序遍历二叉树。

思路及解法

因为昨天刚做过层序遍历二叉树的题102. Binary Tree Level Order Traversal，所以这道题直接用了层序遍历的方法，只是多设置了一个层数的变量，用来判断是否需要反转列表，再将列表存到最后的结果里。
也是分为非递归和递归的方法

代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
         List<List<Integer>> result = new ArrayList<List<Integer>>();
        if(root==null) return result;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int level = 1;
        while(queue.size()!=0){
            int len = queue.size();
            List<Integer> list = new ArrayList<>();
            for(int i=0; i<len; i++){
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left!=null){
                    queue.add(node.left);
                }
                if(node.right!=null){
                    queue.add(node.right);
                }
                
            }
            if(level%2==0) Collections.reverse(list);
            result.add(list);
            level++;
        }
        return result;
    }
}

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        zigzagOrder(result, root, 0);
        return result;
    }
    public void zigzagOrder(List<List<Integer>> result, TreeNode node, int level){
        if(node==null) return;
        if(result.size()<level+1){
            result.add(new ArrayList<Integer>());
        }
        if(level%2==1){
            ((ArrayList<Integer>) result.get(level)).add(0, node.val);
        }else{
            result.get(level).add(node.val);
        }
        zigzagOrder(result, node.left, level+1);
        zigzagOrder(result, node.right, level+1);
        
    }
}