题目
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.

Example 2:

Given tree s:

     3
    / \
   4   5
  / \
 1   2
    /
   0
Given tree t:
   4
  / \
 1   2
Return false.

答案
思路很简单，只要把每个s的node和t比较一下是否为same tree就行

class Solution { 
    public boolean isSameTree(TreeNode p, TreeNode q) {
        if(p == null && q == null)
            return true;
        if((p == null && q != null) || (p != null && q == null) || p.val != q.val)
            return false;       
        return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
    }

    public boolean isSubtree(TreeNode s, TreeNode t) {
        if(s == null) return false;
        boolean ret1 = isSubtree(s.left, t);
        boolean ret2 = isSubtree(s.right, t);
        boolean ret3 = false;
        if(s.val == t.val) {
            ret3 = isSameTree(s, t);
        }
        return ret1 || ret2 || ret3;
    }
}