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.

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.

遍历s所有子树，判断是否和t相同。代码如下：

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