606. 根据二叉树创建字符串C++

https://leetcode.cn/problems/construct-string-from-binary-tree/

class Solution {
public:
	string tree2str(TreeNode* root) {
		if (root == nullptr) return "";
		if (root->left == nullptr && root->right == nullptr)
			return to_string(root->val);
		if (root->right == nullptr) {
			return to_string(root->val)
				+ "(" + tree2str(root->left) + ")";
		}
		return to_string(root->val)
			+ "(" + tree2str(root->left)
			+ ")(" + tree2str(root->right)
			+ ")";
	}
};

class Solution {
public:
	string tree2str(TreeNode* root) {
		string ret;
		if (root == nullptr) return ret;

		ret += to_string(root->val);
		if (root->left || root->right) {
			ret += "(";
			ret += tree2str(root->left);
			ret += ")";
		}

		if (root->right) {
			ret += "(";
			ret += tree2str(root->right);
			ret += ")";
		}
		return ret;
	}
};

改进：
代码二问题：拷贝次数太多

class Solution {
public:
    void _tree2str(TreeNode* root,string& ret)
    {
        if (root == nullptr) return;

		ret += to_string(root->val);
		if (root->left || root->right) {
			ret += "(";
			_tree2str(root->left,ret);
			ret += ")";
		}

		if (root->right) {
			ret += "(";
			_tree2str(root->right,ret);
			ret += ")";
		}
    }

    string tree2str(TreeNode* root) {
        string str;
        _tree2str(root,str);
       return str;
    }
};

采用前序遍历的方式，遍历链表
to_string 可以将数字转化为string