String:606. Construct String from Binary Tree

    递归，为什么我写递归也这么长？？

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    string tree2str(TreeNode* t) {
        stringstream stream;
        string s = "";
        if(t == NULL)
            return s;
        if(t->left != NULL && t->right != NULL)
        {
            stream << t->val;
            s += stream.str();
            s += '(' + tree2str(t->left) + ')';
            s += '(' + tree2str(t->right) + ')';
            
        }
        else if(t->left == NULL && t->right == NULL)
        {
            stream << t->val;
            s += stream.str();
            
        }
        else if(t->left != NULL && t->right == NULL)
        {
            stream << t->val;
            s += stream.str();
            s += '(' + tree2str(t->left) + ')';
            
        }
        else if(t->left == NULL && t->right != NULL)
        {
            stream << t->val;
            s += stream.str();
            s += "()";
            s += '(' + tree2str(t->right) + ')';
        }
        return s;
    }
};

    看别人写的……我的又是垃圾了：

class Solution {
public:
    string tree2str(TreeNode* t) {
        if (t == NULL) return "";
        string s = to_string(t->val);
        if (t->left) s += '(' + tree2str(t->left) + ')';
        else if (t->right) s += "()";
        if (t->right) s += '(' + tree2str(t->right) + ')';
        return s;
    }
};

    int转string用to_string就可以了！