572. Subtree of Another Tree

Given the roots of two binary trees root and subRoot, return true if there is a subtree of root with the same structure and node values of subRoot and false otherwise.

A subtree of a binary tree tree is a tree that consists of a node in tree and all of this node’s descendants. The tree tree could also be considered as a subtree of itself.

帮助方法isEqual，DFS搜索判断两个节点的子节点是否完全相同。
DFS搜索，如果两个根节点的值相等则返回，且调用isEqual方法，如果子节点都相同则返回。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isSubtree(TreeNode root, TreeNode subRoot) {
        return dfs(root, subRoot) != null;
    }
    
    private TreeNode dfs(TreeNode root, TreeNode subRoot){
        if(root == null) return null;
        if(root.val == subRoot.val && isEqual(root, subRoot)) return root;
        if(dfs(root.left, subRoot) != null ) return dfs(root.left, subRoot);
        else return dfs(root.right, subRoot);
    }
    
    private boolean isEqual(TreeNode root, TreeNode subRoot){
        if(root == null || subRoot == null) return root == subRoot;
        if(root.val != subRoot.val) return false;
        return isEqual(root.left, subRoot.left) && isEqual(root.right, subRoot.right);
    }
}