leetcode String to Integer (atoi)

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本文详细解析了atoi函数的实现原理，包括输入参数处理、符号判断、溢出检查及最终转换过程，旨在帮助开发者理解底层逻辑并优化代码。

Implement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

1039 / 1039 test cases passed.

88ms

int atoi(const char *str) {
        if (!str)
            return 0;
       
        bool is_start = false; 
        int max = 2147483647;
        int min = -2147483648;
        int pre_max = 214748364;
        int sign = 1;
        int num = 0;
        const char *p = str;
        while (*p != '\0') {
            char c = *p;
            if (c >= '0' && c <= '9') {
                is_start = true;
                bool overflow = false;
                
                int digit = c - '0';
                if (num - pre_max >= 1)
                    overflow = true;
                else if (num == pre_max) 
                    overflow = digit > 7;
                if (overflow) 
                    return sign > 0 ? max : min;
                num = num * 10 + digit;
            } else {
                if (is_start)
                    break;
                    
                if ('-' == c) {
                    sign = -1;
                    is_start = true;
                } else if ('+' == c) {
                    is_start = true;
                } else if (c != ' ') {
                    return 0;
                }
            }
            ++p;
        }
        
        return sign * num;
    }