[LeetCode]8. String to Integer (atoi)

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.

此題難度不大，主要須要考慮各類邊界條件
1.是否是負數
2.有沒有特殊符號及出現的位置
3.特殊符號是否是加號
4.越界判斷
5.空白的處理

public int myAtoi(String str) {
        str=str.trim();
        int flag=1;
        int index=0;
        int ret=0;
        char[] arr=str.toCharArray();
        int l=arr.length;
        if(l<=0) return 0;
        if(arr[0]=='-'){
            index++;
            flag=-1;
        }else if(arr[0]=='+') index++;
        if(index==l) return 0;
        for(;index<l;index++){
            char c=arr[index];
            if(c<'0' || c>'9')
                if(ret==0) return 0;
                else return ret*flag;
            if(flag==1){
                if(ret>Integer.MAX_VALUE/10) return Integer.MAX_VALUE;
                else if(ret==Integer.MAX_VALUE/10 && c>='8') return Integer.MAX_VALUE;
                ret=ret*10+c-'0';
            }else {
                if(-ret<Integer.MIN_VALUE/10) return Integer.MIN_VALUE;
                else if(-ret==Integer.MIN_VALUE/10 && c>='9') return Integer.MIN_VALUE;
                ret=ret*10+c-'0';
            }
        }
        return ret*flag;
}