【剑指offer】把字符串转换成整数（溢出判断）

class Solution {
public:
    int StrToInt(string str) {
        //关键是判断整数的上下界溢出，利用INT_MIN INT_MAX
        int n = str.size(), s = 1;
        long long res = 0;
        if(!n) return 0;
        if(str[0] == '-') s = -1;
        for(int i = (str[0] ==  '-' || str[0] == '+') ? 1 : 0; i < n; ++i){
            if(!('0' <= str[i] && str[i] <= '9')) return 0;
            if(s == 1 && (res > INT_MAX / 10 || res == INT_MAX / 10 && (str[i] - '0') > INT_MAX % 10)){
                res = 0;
                break;
            }
            if(s == -1 && (res < INT_MIN / 10 || res == INT_MIN / 10 && (str[i] - '0') > -(INT_MIN % 10))){
                res = 0;
                break;
            }//边界条件值得记住
            res = (res << 1) + (res << 3) + s*(str[i] & 0xf);//res=res*10+str[i]-'0';
        }  
        return res;
    }
};