8. String to Integer (atoi)【medium】

解题思路：

遍历字符串然后进行分情况讨论：（ isInit 表示数字是否已经开始，通过 isInit 的值判断是否为开头，如果为 true 表示不是开头）

(1) 空格：如果为开头空格则continue，否则跳出循环

(2) 正负号：如果为开头正负号则设置isNeg的值，否则跳出循环

(3) 数字：将 isInit 置为true，累加结果

(4) 其他符号：跳出循环

考虑边界条件：

当结果溢出时根据正负返回 INT32_MAX 或者 INT32_MIN，所以为了不让中间值溢出，采用 long 类型来保存结果。

实现代码：

// 8. String to Integer (atoi)
int myAtoi(string str) {
  long result = 0;
  bool isInit = false;
  bool isNeg = false;
  for (char c : str) {
    if (c == ' ') {
      if (isInit) {
        break;
      } else {
        continue;
      }
    } else if (c == '-' || c == '+') {
      if (!isInit) {
        isInit = true;
      } else {
        break;
      }
      isNeg = (c == '-');
    } else if (c >= 48 && c <= 57) {
      isInit = true;
      result = result * 10 + (c - 48);
      if (result > INT32_MAX) { 
        return isNeg ? INT32_MIN : INT32_MAX;
      }
    } else {
      break;
    }
  }      
  return (int)(isNeg ? -result : result);
}

问题描述：

Implementatoiwhich 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: [−2^31, 2^31 − 1]. If the numerical value is out of the range of representable values, INT_MAX (2^31 − 1) or INT_MIN (−2^31) 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.