approach 1

push digits & check before overflow

follow the algorithm step by step, pay attention to the difference between positive case and negative case when checking overflow and pushing digit.

C++ code

class Solution {
public:
    int myAtoi(string s) {
        int res = 0, n = s.size(), i = 0;
        bool is_minus = false;

        while (i < n && s[i] == ' ') i++;

        if (i < n) {
            if (s[i] == '-') is_minus = true, i++;
            else if (s[i] == '+') i++;
        }

        while (i < n && s[i] >= '0' && s[i] <= '9') {
            int t = s[i] - '0';
            if (!is_minus) {
                if (res > (INT_MAX - t) / 10) return INT_MAX;
                res = res * 10 + t;
            }else {
                if (res < (INT_MIN + t) / 10) return INT_MIN;
                res = res * 10 - t;
            }
            i++;
        }
        return res;
    }
};