leetcode:Binary Tree Zigzag Level Order Traversal-优快云博客

本文链接：https://blog.youkuaiyun.com/chanrenyuan/article/details/41091705

本文介绍如何使用BFS算法进行遍历，并在遍历过程中每隔一次对数组进行逆序操作，以实现特定的数据排列效果。

简单使用BFS进行遍历，但是每隔一次要逆序一下数组

/**
 * Definition for binary tree
 * 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>> ans = new ArrayList<>();
        if(root != null){
            LinkedList<TreeNode> pre = new LinkedList<>();
            LinkedList<TreeNode> cur = new LinkedList<>();
            pre.add(root);
            List<Integer> list = new ArrayList<>();
            list.add(root.val);
            boolean flag = false;
            do{
                ans.add(list);
                while(!pre.isEmpty()){
                    TreeNode top = pre.pop();
                    if(top.left != null) cur.add(top.left);
                    if(top.right != null) cur.add(top.right);
                }
                list = new ArrayList<>();
                for(TreeNode t : cur){
                    list.add(t.val);
                }
                if(!flag)
                    for(int i = 0, len = list.size() / 2; i < len; ++i){
                        int idx = list.size() - i - 1;
                        int t = list.get(i);
                        list.set(i, list.get(idx));
                        list.set(idx, t);
                    }
                LinkedList<TreeNode> t = cur;
                cur = pre;
                pre = t;
                flag = !flag;
                
            }while(!pre.isEmpty());
        }
        return ans;
    }
}