[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.

题意：找一个树的子树

这里注意一点就行了，必须是子树不能是相同结构的情况

递归判断即可

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