leetcode572. Subtree of Another Tree

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.

解法

递归树s的每一个结点，判断以该结点为根的树和树t是否相等，如果相等，为true，如果不等为false

/**
 * 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 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 != null && t == null) {
            return false;
        }
        if (s.val != t.val) {
            return false;
        }
        return isSame(s.left, t.left) && isSame(s.right, t.right);
    }
}