LeetCode(93) Restore IP Addresses

花了差不多一个晚上找bug，将

string tmpString;
                    tmpString += s[start + 0] + s[start + 1] + s[start + 2];

改为

string tmpString;
            tmpString.push_back(s[start + 0]);tmpString.push_back(s[start + 1]);tmpString.push_back(s[start + 2]);

程序就正常了，什么原因！！！

class Solution {
public:

    void backtracking(string s, int start, int depth, vector<string> &result, string &output, int length) {

        if(start >= length)

            return;

        if(depth == 3) {

            if(length - start > 3)

                return;

            if(length - start == 1) {

                output.push_back(s[start]);
                result.push_back(output);
                output.pop_back();
                return;

            }

            if( length - start == 2) {

                if(s[start] != ‘0‘) {

                    output.push_back(s[start]);output.push_back(s[start + 1]);
                    result.push_back(output);
                    output.pop_back();output.pop_back();
                    return;

                }else {

                    return;

                }

            }

            if( length - start == 3) {

                if(s[start] != ‘0‘) {

                    string tmpString;
                    tmpString.push_back(s[start + 0]);tmpString.push_back(s[start + 1]);tmpString.push_back(s[start + 2]);
                    int tmpInt = stoi(tmpString);
                    if(tmpInt > 255) {

                        return;

                    }else {

                        output.push_back(s[start]);output.push_back(s[start + 1]);output.push_back(s[start + 2]);
                        result.push_back(output);
                        output.pop_back();output.pop_back();output.pop_back();
                        return;

                    }


                }else {

                    return;

                }

            }

        }





        output.push_back(s[start + 0]);output.push_back(‘.‘);

        backtracking(s, start + 1, depth + 1, result, output, length);

        output.pop_back();output.pop_back();


        if((s[start] != ‘0‘) && (length - start >= 2)) {

            output.push_back(s[start + 0]); output.push_back(s[start + 1]);output.push_back(‘.‘);

            backtracking(s, start + 2, depth + 1, result, output, length);

            output.pop_back(); output.pop_back();output.pop_back();

        }

        if((s[start] != ‘0‘) && (length - start >= 3)) {

            string tmpString;
            tmpString.push_back(s[start + 0]);tmpString.push_back(s[start + 1]);tmpString.push_back(s[start + 2]);
            int tmpInt = stoi(tmpString);
            if(tmpInt > 255) {

                return;

            }else {

                output.push_back(s[start + 0]);output.push_back(s[start + 1]);output.push_back(s[start + 2]);output.push_back(‘.‘);

                backtracking(s, start + 3, depth + 1, result, output, length);

                output.pop_back();output.pop_back();output.pop_back();output.pop_back();

            }

        }

    }

    vector<string> restoreIpAddresses(string s) {

        vector<string> result;
        string output;
        int length = s.size();
        backtracking(s, 0, 0, result, output, length);

        return result;

    }

};