### 一、题目

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.
``````

### 二、分析&解题

1. 从左至右扫描，遇到的数字之前的`' '`可以无视
2. 识别符号`+``-`，默认为正数（两个及以上符号时，立即终止）
3. 遇到除了以上三种符号以外的字符，立即终止，返回当前扫描到的整数
4. 没识别到整数时，返回0
5. 转换的范围为32位有符号整数，即`[−2^31, 2^31 − 1]`，超出范围返回边界值

``````Input: "++1"
Output: 0
``````
``````Input: "-001"
Output: -1
``````

#### 思路一：

1. 去除左边的`' '`
2. 发现第一个`'+'``'-'`，则记`foundSymbol = true`，并记录`Symbol`（默认`'+'`
3. 对得到的数字进行累加：`num = num * 10 + currentNum`
4. 最后判断正负号，以及是否超出边界即可

JS实现：

``````/**
* @param {string} str
* @return {number}
*/
var myAtoi = function(str) {
var MAXINT32 = Math.pow(2, 31) - 1;
var MININT32 = -Math.pow(2, 31);
str = str.trimLeft();
var symbol = '+';
var foundSymbol = false;
var counting = false;
var num = 0;
for (var i = 0, len = str.length; i < len; i++) {
if (str[i] === '+' || str[i] === '-') {
if (foundSymbol || counting) break;
foundSymbol = true;
symbol = str[i];
continue;
}
if (str[i] >= '0' && str[i] <= '9') {
num = num * 10 + (str[i] - '0');
counting = true;
continue;
}
break;
}
if (symbol === '-') num = -num;
if (num > MAXINT32) num = MAXINT32;
if (num < MININT32) num = MININT32;
return num;
};
``````

``````if (str[i] === '0' && !counting) {
counting = true;
continue;
}
``````