[Leetcode] String to Integer (atoi)

String to Integer (atoi) 题解

题目来源：https://leetcode.com/problems/string-to-integer-atoi/description/

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Description

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.

Solution

class Solution {
public:
    int myAtoi(string str) {
        int res = 0, i = 0;
        bool positive = true;
        while (str[i] == ' ') i++;
        if (str[i] == '-' || str[i] == '+') {
            positive = str[i] == '+';
            i++;
        }
        while (str[i] >= '0' && str[i] <= '9') {
            if (res > INT_MAX / 10 || (res == INT_MAX / 10 && str[i] - '0' > 7)) {
                return positive ? INT_MAX : INT_MIN;
            }
            res = res * 10 + (str[i] - '0');
            i++;
        }
        return positive ? res : -res;
    }
};

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解题描述

这道题考察的是字符串转数字函数atoi的实现。最主要要解决的几个问题是：

1. 忽略数字前面的空格
2. 正负号判断
3. 溢出判断
4. 非法输入的判断

其中非法输入的判断在上面给出的解中是通过判断字符ASCII范围来判断的。