leetcode-2/code/lc572.java at master · xiaocy/leetcode-2

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package code;

* 572. Subtree of Another Tree

* 题意：判断一棵树是否为另外一棵树的子树

* 难度：Easy

* 分类：Tree

* 思路：两种方法，一种是先序遍历，然后比较字符串即可，注意每个节点开始前加个字符，null也要加进去。

* 另一种递归的方法。

* lc437 lc572 一样的递归思路

* lc297 序列化

* Tips：

public class lc572 {

public class TreeNode {

int val;

TreeNode left;

TreeNode right;

TreeNode(int x) { val = x; }

}

public boolean isSubtree(TreeNode s, TreeNode t) {

if( s==null || t==null ) return s==t;

return helper(s,t) || isSubtree(s.left, t) || isSubtree(s.right, t); // 注意递归方法的不同，是调用哪个函数

}

public boolean helper(TreeNode s, TreeNode t){

if( s==null || t==null ) return s==t;

return s.val==t.val && helper(s.left, t.left) && helper(s.right, t.right);

}

public boolean isSubtree2(TreeNode s, TreeNode t) {

StringBuilder tree1 = new StringBuilder();

StringBuilder tree2 = new StringBuilder();

helper2(s, tree1);

helper2(s, tree2);

return tree1.toString().contains(tree2.toString());

}

public void helper2(TreeNode node, StringBuilder res){

if(node==null)

res.append(",#"); //先放','表示一个新节点的开始，#代表null

else {

res.append( ","+node.val );

helper2(node.left, res);

helper2(node.right, res);

}

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