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.

 

  思路： 

1  写一个判断树是否相同的函数

2 递归的判断当前树节点是否相同。

 

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);
    }
    public 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); 
        
    }
}