String to Integer (atoi)——LeetCode进阶路⑧

原题链接https://leetcode.com/problems/string-to-integer-atoi/

说实话，看到这道题之前，看这通过率有点慌，到底是因为啥 让一道medium的题目这么“厉害”咧，接着往下瞧~

• problem description:

  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.

  实现将字符串转换为整数的atoi。

  函数首先丢弃尽可能多的空格字符，直到找到第一个非空格字符。然后，从这个字符开始，取一个可选的初始加号或减号，后面跟着尽可能多的数字，并将它们解释为一个数值。

  字符串可以在组成整数的字符之后包含其他字符，这些字符将被忽略，并且对该函数的行为没有影响。

  如果str中的非空格字符的第一个序列不是有效的整数，或者由于str为空或只包含空格字符而不存在这样的序列，则不执行转换。

  如果不能执行有效的转换，则返回零值。

  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.

  注意:

  只有空格字符' '被认为是空格字符。

  假设我们正在处理一个环境,只能在32位带符号整数存储整数范围:(231−231−1)。如果数值的范围可表示的值,INT_MAX(231−1)或INT_MIN返回(231−)。

• 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.

• thinking：读罢题目，应该会明确，之所以准确率不高应该与情况较多有关系。
  1.删掉空格
  2.判断是否有正负号，但是只能有一个，否则判为无效转换返回0
  3.一旦遇到非数字，停止转换
  4.出现数据溢出时，返回边界值
• 源码附录：

  class Solution {
      public int myAtoi(String str) {
          str = str.trim();//删空格
  
          if(str == null || str.length() < 1){
              return 0;
          }
  
          int flag = 1;
          int index = 0;
          long result = 0;
  
          if(str.charAt(index) == '-'){
              flag = -1;
              index ++;
          }
          else if(str.charAt(index) == '+'){
              index ++;
          }
  
          for(;index<str.length();index ++){
              int t = str.charAt(index) - '0';
              if(t>9 || t<0){
                 break;
              }
  
              if(flag == 1){
                   result = result*10 + t;
                  if(result > Integer.MAX_VALUE){
                       return Integer.MAX_VALUE;
                  }
              }
              else{
                  result = result*10 - t;
                  if(result < Integer.MIN_VALUE){
                  return Integer.MIN_VALUE;
              }
           }
          }  
  
          int temp = (int)result;
          return temp;
      }
  }

  #四级over 寒假之前被实训和复习安排的明明白白，LeetCode……寒假见