8 字符串转换整数

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xnoilh/

https://leetcode.cn/problems/string-to-integer-atoi/description/

cost: 23m53s

class Solution {
public:
    int myAtoi(string s) {
        int r = 0;
        bool isPositive = true;
        int state = 0; // 0: init, 1: skipping space, 2: parsing +-, 3: skipping
                       // zeros, 4: parsing num
        for (int i = 0; i < s.size();) {
            char ch = s[i];
            if (ch == ' ' && (state == 0 || state == 1)) {
                state = 1;
                i++;
                continue;
            }
            if (ch == '+' && (state == 0 || state == 1)) {
                state = 2;
                i++;
                continue;
            }
            if (ch == '-' && (state == 0 || state == 1)) {
                state = 2;
                isPositive = false;
                i++;
                continue;
            }
            if (ch == '0' && (state <= 3)) {
                state = 3;
                i++;
                continue;
            }
            if (std::isdigit(ch) && (state <= 4)) {
                state = 4;
                if (0x7FFFFFFF / 10 < r) {
                    return isPositive ? 0x7FFFFFFF : -0x7FFFFFFF - 1;
                }
                r *= 10;
                if (0x7FFFFFFF - r < ch - '0') {
                    return isPositive ? 0x7FFFFFFF : -0x7FFFFFFF - 1;
                }
                r += ch - '0';
                i++;
                continue;
            }
            break;
        }
        return isPositive ? r : -r;
    }
};

• 时间复杂度：O(n)
• 空间复杂度：O(1)