leetcode-8-myAtoi

/*
C++版本
*/
class Solution {
public:
    int myAtoi(string s) {
         long long result = 0;

        // 1. 先忽略空格
        int i = 0;
        while( s[i] == ' ' ){ ++i; }

        // 2. 檢查目前字元( + / - / num )
        int pos = 1;
        if(         s[i] == '-'){
            pos = -1; ++i;
        } else if ( s[i] == '+') {
            pos = 1;  ++i;
        } else if ( isdigit(s[i])){
            pos = 1;
        } 
        
        // 3. 遍歷字串
        while( i < s.size() && isdigit(s[i]) ){
            result = result*10 + (s[i]-'0');
            if( !isdigit(s[i]) ) break;
            if( result*pos < INT_MIN ) return INT_MIN;
            if( result*pos > INT_MAX ) return INT_MAX;
            ++i;
        }
        
        // 4. 正負號
        result = result*pos;

        // 5. 檢查大小
        if( result < INT_MIN ) result = INT_MIN;
        if( result > INT_MAX ) result = INT_MAX;
        return result;

    }
};