leetcode

题目

Given two non-empty binary trees s andt, check whether tree t has exactly the same structure and node values with a subtree ofs. A subtree of s is a tree consists of a node ins 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.

题意

给定两个非空的二叉树s , t 。判断t 是否准确存在于s中（结构和值都相同），即是s的子树。此处的子树定义为，s的一个结点以及该结点的所有后代。（s本身也可以看做自己的后代）

分析及解答

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);    }}