Leetcode 8. String to Integer (atoi) atoi函数实现 (字符串)

题目描述

实现atoi函数，将一个字符串转化为数字

测试样例

Input: "42"

Output: 42

Input: "   -42"

Output: -42

Input: "4193 with words"

Output: 4193

Input: "words and 987"

Output: 0

详细分析

这道题的corner cases非常多，请务必确保下面cases都能通过的情况下再提交。

"42"

"words and 987"

"-91283472332"

"0-1"

"-000000000000001"

"  0000000000012345678"

"10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000522545459"

"-2147483647"

"-2147483648"

"2147483648"

"2147483649"

""

"7"

"   +0 123"

算法实现

class Solution {

public:

    string trim(const std::string&str){

        string nstr;

        int i=0;

        while(isspace(str[i])){

            i++;

        }

        for(;i<str.length();i++){

            if(isspace(str[i])){

                break;

            }

            nstr +=str[i];

        }

        return nstr;

    }

    int myAtoi(string str) {

        str = trim(str);

        if(str.length()==0 || (str[0]!='+'&&str[0]!='-'&& !isdigit(str[0]))){

            return 0;

        }

        int i=0;

        //consume sign char

        if(str[0] =='+' || str[0]=='-'){

            i++;

        }

        string nstr;

        while(isdigit(str[i])){

            nstr+=str[i];

            i++;

        }

        if(nstr.length()==0){

            return 0;

        }

        i=0;

        // consume meaningless zeros

        while(nstr[i]=='0'){

            i++;

        }

        nstr = nstr.substr(i);

        long long result = 0L;

        unsigned long long exp = 1;

        for(int k=nstr.length()-1;k>=0;k--){

            result += ((int)(nstr[k]-'0'))*exp;

            if(exp> numeric_limits<int>::max()){

                return str[0]=='-'?numeric_limits<int>::min():numeric_limits<int>::max();

            }

            exp*=10;

            if(result> numeric_limits<int>::max()){

                return str[0]=='-'?numeric_limits<int>::min():numeric_limits<int>::max();

            }

        }

        return str[0]=='-'?-result:result;

    }

};