面試中遇到一道演算法題：不採用java的內建函式，把String型別轉換為int型別。現採用兩種方法實現（直接上程式碼）：

package com.haiwi.test;

public class AtoiTest {
  public static void main(String[] args) throws Exception {
    String s = "-011134";
    System.out.println("轉換前的字串：" + s);
    System.out.println("atoi1轉換後的字串：" + atoi1(s));
    System.out.println("atoi2轉換後的字串："
 
 + atoi2(s));

  }

  /**
   * 不用java內建函式，將String字串轉換為數字
   * @param s
   * @return
   * @throws Exception 
   */
  public static int atoi1(String s) throws Exception {
    if (s == null || s.length() == 0) {
      throw new Exception("要轉換的字串為空，無法轉換！");
    }
    int retInt = 0;
    int[] num = new int[s.length()];
    for
 
 (int i = 0; i < s.length(); i++) {
      char c = s.charAt(i);
      switch (c) {
      case '-':
        num[i] = -1;
        break;
      case '0':
        num[i] = 0;
        break;
      case '1':
        num[i] = 1;
        break;
      case '2':
        num[i] = 2;
        break;
      case '3'
 
:
        num[i] = 3;
        break;
      case '4':
        num[i] = 4;
        break;
      case '5':
        num[i] = 5;
        break;
      case '6':
        num[i] = 6;
        break;
      case '7':
        num[i] = 7;
        break;
      case '8':
        num[i] = 8;
        break;
      case '9':
        num[i] = 9;
        break;
      default:
        throw new Exception("要轉換的字串格式錯誤，無法轉換！");
      }
    }
    for (int i = 0; i < num.length; i++) {
      if (num[i] < 0 && i > 0) {
        throw new Exception("要轉換的字串格式錯誤，無法轉換！");
      }
      if (num[i] < 0) {
        continue;
      }
      retInt += Math.pow(10, num.length - i - 1) * num[i];
    }
    if (num[0] == -1) {//代表負數
      retInt = -retInt;
    }
    return retInt;
  }
  /**
   * 不用java內建函式，將String字串轉換為數字
   * @param s
   * @return
   * @throws Exception
   */
  public static int atoi2(String s) throws Exception{
    int retInt = 0;
    if (s == null || s.length() == 0) {
      throw new Exception("要轉換的字串為空，無法轉換！");
    }
    boolean isNegative = false;
    for (int i = 0; i < s.length(); i++) {
      if (i==0) {
        if(s.charAt(i)=='-'){
          isNegative = true;
          continue;
        }
      }else{
        if(s.charAt(i)>'9' || s.charAt(i)<'0'){
          throw new Exception("要轉換的字串格式錯誤，無法轉換！");
        }
      }
      retInt *=10;
      retInt += s.charAt(i) - '0';
    }
    return isNegative ? -retInt : retInt;
  }
}