LeetCode8. String to Integer (atoi)（模拟 / 思维）

LeetCode8. String to Integer (atoi)（模拟 / 思维）

• Medium
• Accepted：348,594
• Submissions：2,391,280

Implement atoi which converts a string to an integer.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

If no valid conversion could be performed, a zero value is returned.

Note:

• Only the space character ' ' is considered as whitespace character.
• Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.

Example 1:

Input: "42"
Output: 42

Example 2:

Input: "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
             Then take as many numerical digits as possible, which gets 42.

Example 3:

Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.

Example 4:

Input: "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical 
             digit or a +/- sign. Therefore no valid conversion could be performed.

Example 5:

Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
             Thefore INT_MIN (−231) is returned.

链接

https://leetcode.com/problems/string-to-integer-atoi

题意

给定一个字符串，将其转化为整数，相当于实现atoi函数，有很多规则。

题解

参考自：My simple solution

1. discards all leading whitespaces
2. sign of the number
3. overflow
4. invalid input

解释一下base == INT_MAX / 10 && str[i] > '7'，如果之前得到的数为214748364然后当前数字字符大于7那么说明这轮之后base会超出整型范围所以直接返回。

别的看代码就能懂：）

代码

• Runtime: 8 ms, faster than 99.69% of C++ online submissions for String to Integer (atoi).
• Memory Usage: 8.6 MB, less than 99.80% of C++ online submissions for String to Integer (atoi).

class Solution {
public:
    int myAtoi(string str) {
        if (str.empty())
            return 0;
        int sign = 1, base = 0, i = 0;
        while (str[i] == ' ')
            ++i;
        if (str[i] == '+' || str[i] == '-')
            sign = 1 - 2 * (str[i++] == '-' ? 1 : 0);
        while (str[i] >= '0' && str[i] <= '9')
        {
            if (base > INT_MAX / 10 || (base == INT_MAX / 10 && str[i] > '7'))
                return (sign == 1) ? INT_MAX : INT_MIN;
            base = base * 10 + (str[i++] - '0');
        }
        return base * sign;
    }
};

The end.
2019年4月16日 星期二