leetcode 572. Subtree of Another Tree（判断子树）-爱代码爱编程

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.

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.

给出两棵二叉树s和t， 判断t是不是s的子树。

思路：
用到了相同树的判断，如果不是相同的树，就进一步判断t 在s的左子树，右子树中有没有相同的树

    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);
    }
    
    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 isSame(s.left, t.left) && isSame(s.right, t.right);
        return false;
    }