String to Integer (atoi)
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.
Requirements for atoi:
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. If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) 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: "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 4:
Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
Example 5:
Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
Thefore INT_MIN (−2^31) is returned.
思路
细节题。注意以下的测试用例
- 不规则输入
- 无效格式
- 溢出数据
class Solution {
public:
int myAtoi(const string &str) {
int num = 0;
int sign = 1;
const int n = str.length();
if (n == 0) return 0;
int i = 0;
while (str[i] == ' ' && i < n) i++;
if (str[i] == '+') {
i++;
} else if (str[i] == '-') {
sign = -1;
i++;
}
for (; i < n; i++) {
if (str[i] < '0' || str[i] > '9')
break;
if (num > INT_MAX / 10 ||
(num == INT_MAX / 10 &&
(str[i] - '0') > INT_MAX % 10)) {
return sign == -1 ? INT_MIN : INT_MAX;
}
num = num * 10 + (str[i] - '0');
}
return num * sign;
}
};