String to Integer (atoi)

最新推荐文章于 2019-08-31 13:11:50 发布

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本文介绍了一个使用简单状态机实现字符串转换为整数的方法（atoi）。重点在于处理各种输入情况，包括正负号、非数字字符及溢出判断。

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.

spoilers alert... click to show requirements for atoi.

一个简单状态机实现，值得注意的是溢出值得判定和返回。

class Solution {
public:
    int atoi(const char *str) {
        if (str == NULL) throw -1;
        int ret = 0, state = 0;
        bool isneg = false;
        int maxbar = INT_MAX / 10;
        int maxextra = INT_MAX % 10;
        for (int i = 0; str[i] != '\0'; i++) {
            char cc = str[i];
            switch (state) {
            case 0:
                if (cc == '+' || cc == '-' || isdigit(cc)) {
                    state = 1;
                    if (cc == '-') isneg = true;
                    if (isdigit(cc)) i--;
                } else if (cc != ' ') return ret;
                break;
            case 1:
                if (!isdigit(cc)) {
                    return isneg ? -ret : ret; 
                }
                
                {            
                    int cval = cc - '0';
                    if (ret > maxbar || (ret == maxbar && cval > maxextra)) {
                        return isneg ? INT_MIN : INT_MAX;
                    }
                    
                    ret *= 10;
                    ret += cval;
                }
                break;
            default:
                return isneg ? -ret : ret;
            }
        }
        
        return isneg ? -ret : ret;
    }
};