8. String to Integer (atoi) (重要)

atoi

Hint:

Notes:

Update (2015-02-10):

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注意5点。

1.空字符串

2.字符串前的空格

3.+-号

4.非法字符（这里返回非法字符前的有效位）

5.溢出（这里如果大于最大，返回最大；小于最小返回最小）

class Solution {
public:
  int myAtoi(string str) {
    int len = str.size();
    if (len == 0) return 0;

    long long sum = 0;
    int i = 0;
    while (str[i] == ' ') i++;

    int minus = 1;
    if (str[i] == '-'){
      minus = -1;
      i++;
    }
    else if (str[i] == '+'){
      i++;
    }

    for (; i < len; i++){
      if (str[i]<'0' || str[i]>'9'){
        break;
      }
      sum = sum*10 + (str[i] - '0')*minus;
      if (sum>INT_MAX){
        return INT_MAX;
      }
      else if (sum < INT_MIN){
        return INT_MIN;
      }
    }
    return sum;
  }
};