Leetcode572 Subtree of Another Tree

题目描述

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.

思路

通过递归isSubtree比较是否为s树的子树，在寻找子树过程中，通过调用isSame函数去比较两颗树的左子树和右子树的值是否相同。

代码

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