题目描述

请考虑一颗二叉树上所有的叶子，这些叶子的值按从左到右的顺序排列形成一个 叶值序列 。
872. 叶子相似的树

解法1 迭代

前序遍历得到二叉树的叶值序列，然后将它们比较即可。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean leafSimilar(TreeNode root1, TreeNode root2) {
        if (root1 == null || root2 == null) {
            return false;
        }
        List<Integer> list1 = new ArrayList<>();
        List<Integer> list2 = new ArrayList<>();
        list1 = helper(root1);
        list2 = helper(root2);
        if (list1.equals(list2)) {
            return true;
        }
        return false;
    }

    public List<Integer> helper(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        LinkedList<TreeNode> stack = new LinkedList<>();

        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                if (root.left == null && root.right == null) {
                    list.add(root.val);
                }
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            root = root.right;
        }
        return list;
    }
}

解法2 递归

上面的思路，用递归实现了一遍

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean leafSimilar(TreeNode root1, TreeNode root2) {
        if (root1 == null || root2 == null) {
            return false;
        }
        List<Integer> list1 = new ArrayList<>();
        List<Integer> list2 = new ArrayList<>();
        helper(root1, list1);
        helper(root2, list2);
        if (list1.equals(list2)) {
            return true;
        }
        return false;
    }

    public void helper(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        if (root.left == null && root.right == null) {
            list.add(root.val);
        }
        helper(root.left, list);
        helper(root.right, list);
    }
}