String to Integer (Leetcode 8)

Question

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.

Update (2015-02-10): The signature of the C++ function had been updated. If you still see your function signature accepts a const char * argument, please click the reload button to reset your code definition.

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

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.

Solution

这道题不难，就是将string转化成整数。但是要考虑的特殊情况。

1. 首先要将开始的所有空白字符去掉

2. 记录正负号。如果第一个不是空白的字符为“-”，则记录为负，否则记录为正

3. 然后一位一位加起来即可。为了防止越界，用double来记结果

4. 考虑最大值和最小值的情况

代码如下：

public class Solution {
    public int myAtoi(String str) {
        if(str == null || str.length() == 0){
            return 0;
        }

        str = str.trim();

        int start = 0;
        boolean isNegative = false;
        if(str.charAt(0) == '-'){
            isNegative = true;
            start++;
        }else if(str.charAt(0) == '+'){
            start++;
        }

        double res = 0;
        while(start < str.length() && Character.isDigit(str.charAt(start))){
            int temp = str.charAt(start++) - '0';
            res = res * 10 + temp;
        }

        if(isNegative){
            res = -res;
        }

        if(isNegative){
            return (int)Math.max(Integer.MIN_VALUE, res);
        }else{
            return (int)Math.min(Integer.MAX_VALUE, res);
        }
    }
}