【LeetCode】872. Leaf-Similar Trees

Description:

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.

Note:

• Both of the given trees will have between 1 and 100 nodes.

分析：此题需要分别获取两个二叉树所有叶子节点的值，可存入list进行比较。题解用前序遍历来获取所有叶子节点。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> a = new ArrayList<Integer>();
    List<Integer> b = new ArrayList<Integer>();
    public boolean leafSimilar(TreeNode root1, TreeNode root2) {
        getAllChild(root1,1);
        getAllChild(root2,2);
        if(a.size()!=b.size())
            return false;
        for(int i = 0; i < a.size(); i++){
            if(a.get(i)!=b.get(i))
                return false;
        }
        return true;
    }
    //前序遍历获取所有叶子节点
    public void getAllChild(TreeNode root,int number){
        if(root != null){
            if(root.left == null && root.right == null){
                if(number==1)
                    a.add(root.val);
                else
                    b.add(root.val);
            }
            getAllChild(root.left,number);
            getAllChild(root.right,number);
        }
    }
}

这个方法可以AC，但运行时间只超过了23%的java提交。

看了下别人的方法，用字符串替代了list，运行时间超过100%的java提交。

参考代码如下：

/**
 * 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) {
        String leaf1 = helper(root1);
        String leaf2 = helper(root2);
        
        return leaf1.equals(leaf2);
    }
    
    private String helper(TreeNode root) {
        if(root == null) {
            return "";
        }
        if(root.left == null && root.right == null) {
            return ""+root.val;
        }
        String left = helper(root.left);
        String right = helper(root.right);
        return left+right;
    }
}