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.

Examples

Example 1:

Input: root = [3,4,5,1,2], subRoot = [4,1,2]
Output: true

Example 2:

Input: root = [3,4,5,1,2,null,null,null,null,0], subRoot = [4,1,2]
Output: false

Constraints:

• The number of nodes in the root tree is in the range [1, 2000].
• The number of nodes in the subRoot tree is in the range [1, 1000].
• -10^4 <= root.val <= 10^4
• -10^4 <= subRoot.val <= 10^4

解析

每個兩個樹的根結點root, subRoot

題目要實作一個演算法去察看 subRoot 是不是 root 的 subTree

subRoot 是 root 的 subTree 有以下幾種狀況

1. subRoot 的 tree 與 root 的 tree 相等
2. subRoot 的 tree 是 root.left tree 的 subTree
3. subRoot 的 tree 是 root.right tree 的 subTree

要檢驗 subRoot tree與 root tree是否相等有幾個條件

1 root 是空的時候 , subRoot 必須是空

2 root 非空， root 的 Tree 與 subRoot 的 Tree 相等

2 root 非空, subRoot 的 Tree 是 root.left Tree 的 subTree

3 root 非空, subRoot 的 Tree 是 root.right Tree 的 subTree

程式碼

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func isSubtree(root *TreeNode, subRoot *TreeNode) bool {
   if root == nil {
      return subRoot == nil
   }
   return IsEqual(root, subRoot)||isSubTree(root.Left, subRoot)||isSubTree(root.Right, subRoot)
}

func IsEqual(root *TreeNode, subRoot *TreeNode) bool {
	 if root == nil {
      return subRoot == nil
   }
   if subRoot != nil {
      if root.Val != subRoot.Val {
	      return false
      }
      return IsEqual(root.Left, subRoot.Left)&&IsEqual(root.Right, subRoot.Right)
   } else {
      return false
   }
}

困難點

1. 要明白某一個樹是另一個樹的 subTree 的定義
2. 要理解怎麼去檢驗兩樹相等

Solve Point

• [x] Understand what problem would like to solve
• [x] Analysis Complexity