LeetCode 572 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.
也就是说给两个树 判断第二棵树是不是第一棵树的子树。

判断是不是子树 就跟之前的判断两个树一否完全一样相同，但是又有一些区别 区别在哪里呢？暂时想不明白

isSubtree(s, t) = isSubtree(s.left, t) || isSubtree(s.right, t)

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