[AcWing 87] 把字符串转换成整数

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class Solution {

public:

    int strToInt(string str) {

        int k = 0;

        while (k < str.size() && str[k] == ' ')     k++;

        long long res = 0;

        int minus = 1;

        if (k < str.size()) {

            if (str[k] == '-')      minus *= -1, k++;

            else if (str[k] == '+')    k++;

        }

        while (k < str.size() && str[k] >= '0' && str[k] <= '9') {

            res = res * 10 + str[k] - '0';

            if (res > 1e11)     break;

            k++;

        }

        res *= minus;

        if (res > INT_MAX)  res = INT_MAX;

        if (res < INT_MIN)  res = INT_MIN;

        return res;

    }

};