算法题：String类型转int类型(不用Java内置函数)

如何不采用java的内置函数，把String类型转换为int类型，想到两种方法，如下代码自己测试下

package com.journey.test;


public class AtoiTest {

  public static void main(String[] args) throws Exception {

    String s = "-2233113789";

    System.out.println("转换前的字符串：  " + s);

    System.out.println("atoi1转换后的字符串：" + atoi1(s));

    System.out.println("atoi2转换后的字符串：" + atoi2(s));


  }

方法一：遍历检索法，遍历字符串，charAt() 方法用于返回指定索引处的字符，取出字符对照0-9的数字。

  /**

   * 不用java内置函数，将String字符串转换为数字

   * @param s

   * @return

   * @throws Exception 

   */

  public static int atoi1(String s) throws Exception {

    if (s == null || s.length() == 0) {

      throw new Exception("要转换的字符串为空，无法转换！");

    }

    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("要转换的字符串格式错误，无法转换！");

      }

    }

    for (int i = 0; i < num.length; i++) {

      if (num[i] < 0 && i > 0) {

        throw new Exception("要转换的字符串格式错误，无法转换！");

      }

      if (num[i] < 0) {

        continue;

      }

      retInt += Math.pow(10, num.length - i - 1) * num[i];

    }

    if (num[0] == -1) {//代表负数

      retInt = -retInt;

    }

    return retInt;

  }

方法二：判断字符是否在 范围 s.charAt(i)>'9' || s.charAt(i)<'0'

  /**

   * 不用java内置函数，将String字符串转换为数字

   * @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("要转换的字符串为空，转换失败！");

    }

    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("要转换的字符串格式错误，转换失败！");

        }

      }

      retInt *=10;

      retInt += s.charAt(i) - '0';

    }

    return isNegative ? -retInt : retInt;

  }

}