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.

先判断s当前节点的子树和t是否是同一个树；如果不是，递归判断t是否是s.left/s.right的子树；

time: O(m*n)　　-- worst case: if both tree are skew tree, average: O(nlogn)

space: O(logn)　　-- worst case: O(n)

/**
 * 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 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);
    }
}