String to Integer (atoi)
package cn.edu.xidian.sselab;
import java.util.HashMap;
/**
* title:String to Integer (atoi)
* content:
* Implement atoi to convert a string to an integer.
* Hint: Carefully consider all possible input cases. If you want a challenge,
* please do not see below and ask yourself what are the possible input cases.
* Notes: It is intended for this problem to be specified vaguely (ie, no given input specs).
* You are responsible to gather all the input requirements up front.
*/
public class StringToInteger {
//错误的解法,自己没有想明白这个问题,以为任意的字符都可以转换成整型,想的太天真,其实这个问题,自己看Java源码的时候看过Integer.parseInt(String str)
public static int myAtoi(String str){
StringBuffer sb = new StringBuffer();
if(str == null){
return Integer.MIN_VALUE;
}else if(str.equals("")){
return 0;
}
else{
int len = str.length();
int j = 0;
if(str.startsWith("-")||str.startsWith("+")){
sb.append(str.charAt(0));
j++;
}
for(int i=j;i<len;i++){
char temp = str.charAt(i);
if(temp >= '0' && temp <= '9'){
sb.append(temp);
}else if(temp >= 'A' && temp <= 'Z'){
sb.append(65 + temp - 'A');
}else if((temp >= 'a' && temp <= 'z')){
sb.append(97 + temp - 'a');
}else {
sb.append("0");
}
}
}
System.out.println(sb);
int num = Integer.valueOf(sb.toString());
if(num > Integer.MAX_VALUE){
return Integer.MAX_VALUE;
}else if(num < Integer.MIN_VALUE){
return Integer.MIN_VALUE;
}else{
return num;
}
}
//正确的解法
//这个问题要注意几个方面(1)输入是“”,null这种字符串的时候,返回0
//(2)判断转换的值是否大于Integer的最大值,是否小于Integer的最小值,及超过int的范围,返回int最大值或最小值
//(3)里面会有“+”“-”的影响
//学到了一个API方法Character.isWhitespace()判断是否为“”
public static int myAtois(String str){
int p = 0 ;
int ret = 0;
int digit = 0;
int len = str.length();
if(str == null || len == 0)
return 0;
while(p < len && Character.isWhitespace(str.charAt(p)))
p++;
if(p == len)
return 0;
boolean flag = (str.charAt(p) == '-');
if(str.charAt(p) == '-' || str.charAt(p) == '+'){
p++;
}
for(;p<len;p++){
if(str.charAt(p) > '9'||str.charAt(p) < '0'){
break;
}else{
digit = str.charAt(p) - '0';
if(!flag && ((Integer.MAX_VALUE - digit) / 10 < ret))
return Integer.MAX_VALUE;
if(flag && ((Integer.MIN_VALUE + digit) / 10 > ret))
return Integer.MIN_VALUE;
ret = ret * 10 + (flag? -digit : digit);
}
}
return ret;
}
}