[LeetCode] 872. Leaf-Similar Trees 葉結點相似的樹

Consider all the leaves of a binary tree.  From left to right order, the values of those leaves form a *leaf value sequence.*

For example, in the given tree above, the leaf value sequence is (6, 7, 4, 9, 8).

Two binary trees are considered leaf-similar if their leaf value sequence is the same.

Return true if and only if the two given trees with head nodes root1 and root2 are leaf-similar.

這道題定義了一種葉相似樹，就是說若兩棵樹的葉結點按照從左向右的順序取出來排成序列，若兩個序列相同，則說明二者是葉結點相似樹。其實本質就是按從左到右的順序打印二叉樹的葉結點唄，那么根據這種順序，我們采用先序遍歷遍歷比較好，遇到葉結點后直接將葉結點存入數組中，那么對於兩個樹遍歷后就分別得到兩個包含葉結點的數組，最后再比較一下這兩個數組是否相同即可，參見代碼如下：
解法一：

class Solution {
public:
    bool leafSimilar(TreeNode* root1, TreeNode* root2) {
		vector<int> leaf1, leaf2;
		helper(root1, leaf1);
		helper(root2, leaf2);
		return leaf1 == leaf2;
    }
	void helper(TreeNode* node, vector<int>& leaf) {
		if (!node) return;
		if (!node->left && !node->right) {
			leaf.push_back(node->val);
		}
		helper(node->left, leaf);
		helper(node->right, leaf);
	}
};

我們也可以不用數組，而是用兩個字符串，那么在每個葉結點值直接要加上一個分隔符，這樣才能保證不會錯位，最后比較兩個字符串是否相等即可，參見代碼如下：
解法二：

class Solution {
public:
    bool leafSimilar(TreeNode* root1, TreeNode* root2) {
		string leaf1, leaf2;
		helper(root1, leaf1);
		helper(root2, leaf2);
		return leaf1 == leaf2;
    }
	void helper(TreeNode* node, string& leaf) {
		if (!node) return;
		if (!node->left && !node->right) {
			leaf += to_string(node->val) + "-";
		}
		helper(node->left, leaf);
		helper(node->right, leaf);
	}
};

Github 同步地址:

https://github.com/grandyang/leetcode/issues/872

類似題目：

Binary Tree Preorder Traversal

參考資料：

https://leetcode.com/problems/leaf-similar-trees/

https://leetcode.com/problems/leaf-similar-trees/discuss/152329/C%2B%2BJavaPython-O(logN)-Space

https://leetcode.com/problems/leaf-similar-trees/discuss/152358/Simple-6-lines-Java-StringBuilder-%2B-traverse-with-explanation

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