297 二叉树的序列化与反序列化

https://leetcode.cn/leetbook/read/top-interview-questions-medium/xwxa3m/

cost:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if (root == nullptr) {
            return ",null";
        }
        string s = "," + to_string(root->val);
        s += this->serialize(root->left);
        s += this->serialize(root->right);
        return s;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string s) {
        int idx = 0;
        return this->deserialize(s, idx);
    }

private:
    TreeNode* deserialize(string& s, int& idx) {
        auto node = this->parseTreeNode(s, idx);
        if (node == nullptr) {
            return nullptr;
        }
        node->left = this->deserialize(s, idx);
        node->right = this->deserialize(s, idx);
        return node;
    }

    TreeNode* parseTreeNode(string& s, int& idx) {
        bool valid = false;
        int r = 0;
        bool positive = true;
        while (idx < s.size()) {
            if (s[idx] == ',') {
                idx++;
                continue;
            }
            if (s[idx] == 'n') {
                idx += 4;
                return nullptr;
            }
            valid = true;
            if (s[idx] == '-') {
                positive = false;
                idx++;
                continue;
            }
            while (idx < s.size() && s[idx] >= '0' && s[idx] <= '9') {
                r *= 10;
                r += s[idx] - '0';
                idx++;
            }
            break;
        }
        if (!valid) {
            return nullptr;
        }
        if (!positive) {
            r = -r;
        }
        return new TreeNode(r);
    }
};

// Your Codec object will be instantiated and called as such:
// Codec ser, deser;
// TreeNode* ans = deser.deserialize(ser.serialize(root));