Problem

Given two non-empty binary treessandt, check whether treethas exactly the same structure and node values with a subtree ofs. A subtree ofsis a tree consists of a node insand all of this node's descendants. The treescould 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.

错误思路

• 直接就想用isSubtree来解决全部。 实际上要从逻辑去想问题，而非一脑子地去用某个固化的方式去解决

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

正确思路

• 在上面的基础上更改
• 如果当前s.val == t.val， 那么他们的左子树相同，右子树也相同

    public boolean isSubtree(TreeNode s, TreeNode t) {
        if (s == null && t == null) return true;
        if (s == null || t == null) return false;
        if (s.val == t.val && isSame(s.left, t.left) && isSame(s.right, t.right)) 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 && isSame(s.left, t.left) && isSame(s.right, t.right)) return true;
        return false;
    }