[LeetCode][Tree] 572. Subtree of Another Tree.md

Description

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.

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isSubtree(TreeNode s, TreeNode t) {
        if (s == null) return t == null;
        return isEqual(s, t) || isSubtree(s.left, t) || isSubtree(s.right, t);
    }

    private boolean isEqual(TreeNode s, TreeNode t) {
        if (s == null && t == null) return true;
        if (s == null || t == null) return false;
        
        if (t.val != s.val) return false;
        return isEqual(s.left, t.left) && isEqual(s.right, t.right);
    }
}

扩展：如果此题要求Example 2也返回 true 的话，可以在 iSEqual 中判断如果 t==null ，就返回 true 

变种

leetcode 572要求了t不为null https://leetcode-cn.com/problems/shu-de-zi-jie-gou-lcof/