572-Subtree of Another Tree

Description

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.

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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.

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問題描述

給定非空二叉樹s和t， 判斷t是否為s的子樹， 即t是否與s中的某一個子樹有相同的結構和對應的值。s的子樹為包含該子樹根節點及其所有子節點的樹。s也可以看作是自己的子樹。

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問題分析

遍歷s的每個節點， 判斷其是否與t相同

isSubtree()用來遍歷， isSame()用來判斷兩個樹是否相同。

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解法

class Solution {
    public boolean isSubtree(TreeNode s, TreeNode t) {
        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;
        if(s.val != t.val) return false;
        return isSame(s.left, t.left) && isSame(s.right, t.right);
    }
}