LeetCode 8 String to Integer (string转int)

题目来源：https://leetcode.com/problems/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.

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.

 

解题思路：

照着要求写代码，可以总结如下：

1. 字串为空或者全是空格，返回0； 

2. 字串的前缀空格需要忽略掉；

3. 忽略掉前缀空格后，遇到的第一个字符，如果是‘+’或‘－’号，继续往后读；如果是数字，则开始处理数字；如果不是前面的2种，返回0；

4. 处理数字的过程中，如果之后的字符非数字，就停止转换，返回当前值；

5. 在上述处理过程中，如果转换出的值超出了int型的范围，就返回int的最大值或最小值。

 

Java代码：

public class Solution {
    public int myAtoi(String str) {
        int max = Integer.MAX_VALUE;
        int min = -Integer.MIN_VALUE;
        long result = 0;
        str = str.trim();
        int len = str.length();
        if (len < 1)
            return 0;
        int start = 0;
        boolean neg = false;
 
        if (str.charAt(start) == '-' || str.charAt(start) == '+') {
            if (str.charAt(start) == '-')
                neg = true;
            start++;
        }
 
        for (int i = start; i < len; i++) {
            char ch = str.charAt(i);
 
            if (ch < '0' || ch > '9')
                break;
            result = 10 * result + (ch - '0');
            if (!neg && result > max)
                return max;
            if (neg && -result < min)
                return min;
 
        }
        if (neg)
            result = -result;
 
        return (int) result;
    }
 
};