572. Subtree of Another Tree

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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:

1
2
3
4
5
6
7
8
9
10
11
    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:

1
2
3
4
5
6
7
8
9
10
11
12
13
    3
   / \
  4   5
 / \
1   2
   /
  0
Given tree t:
  4
 / \
1   2

Return false.

解法1：

很简单的一个递归算法。如果当前节点数值一样，那么先判断是否identical,如果不是，考虑left, 考虑right是否和target tree存在subset关系。

C++

1

Java

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSubtree(TreeNode s, TreeNode t) {
        if (s == null && t == null) {
            return true;
        } else if (s == null || t == null) {
            return false;
        }
        if (s.val == t.val) {
            if (identical(s, t)) {
                return true;
            }
        }
        return isSubtree(s.left, t) || isSubtree(s.right, t);
    }
    private boolean identical(TreeNode s, TreeNode t) {
        if (s == null && t == null) {
            return true;
        } else if (s == null || t == null) {
            return false;
        }
        if (s.val != t.val) {
            return false;
        }
        return identical(s.left, t.left) && identical(s.right, t.right);
    }
}