LeetCode: Subtree of Another Tree

Given two non-empty binary trees s and t, check whether tree t has exactly the same structure and node values with a subtree of s. A subtree of s is a tree consists of a node in s and all of this node's descendants. The tree s could also be considered as a subtree of itself.

给两棵非空的二叉树s和t，检查二叉树t是否为s的子树（树t由树s中的一个结点及其所有子孙结点组成）

例如：

Example 1:
Given tree s:

     3
    / \
   4   5
  / \
 1   2

Given tree t:

   4 
  / \
 1   2

Return true, because t has the same structure and node values with a subtree of s.

Example 2:
Given tree s:

     3
    / \
   4   5
  / \
 1   2
    /
   0

Given tree t:

   4
  / \
 1   2

Return false.

解题思路：使用二叉树的先序遍历的思想，对于s，检查以其中每个结点为根节点的子树是否与t相同

代码如下：

public class Solution {
    public boolean isSubtree(TreeNode s, TreeNode t) {
        if (s == null) return false;
        if (isSame(s, t)) return true;
        return isSubtree(s.left, t) || isSubtree(s.right, t);
    }
    
    private boolean isSame(TreeNode s, TreeNode t) {
        if (s == null && t == null) return true;
        if (s == null || t == null) return false;
        
        if (s.val != t.val) return false;
        
        return isSame(s.left, t.left) && isSame(s.right, t.right);
    }
}