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[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.

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.

另一个树的子树。

题意是给两个树s和t，判断t是否是s的子树。如果s和t相同，也可以视为t是s的子树。

这个题的思路跟100题same tree非常像。既然是问t是否是s的子树，那也就意味着s这棵树比t要大，或者最多相同，所以就可以按照100题的思路递归比较

s的左子树和右子树是否分别和t一样
s是否和t是相同的树

时间O(n)

空间O(n)

Java实现

 1 class Solution {
 2     public boolean isSubtree(TreeNode s, TreeNode t) {
 3         if (s == null) {
 4             return false;
 5         }
 6         if (isSame(s, t)) {
 7             return true;
 8         }
 9         return isSubtree(s.left, t) || isSubtree(s.right, t);
10     }
11 
12     private boolean isSame(TreeNode s, TreeNode t) {
13         if (s == null && t == null) {
14             return true;
15         }
16         if (s == null || t == null) {
17             return false;
18         }
19         if (s.val != t.val) {
20             return false;
21         }
22         return isSame(s.left, t.left) && isSame(s.right, t.right);
23     }
24 }

JavaScript实现

 1 /**
 2  * @param {TreeNode} s
 3  * @param {TreeNode} t
 4  * @return {boolean}
 5  */
 6 var isSubtree = function (s, t) {
 7     if (s == null) {
 8         return false;
 9     }
10     if (isSameTree(s, t)) {
11         return true;
12     }
13     return isSubtree(s.left, t) || isSubtree(s.right, t);
14 };
15 
16 var isSameTree = function (s, t) {
17     if (s && t) {
18         return s.val === t.val && isSameTree(s.left, t.left) && isSameTree(s.right, t.right);
19     }
20     return s === t;
21 }