剑指offer 面试题49 把字符串转换成整数

剑指offer 面试题49 把字符串转换成整数

package algorithm.foroffer.top50;

import org.junit.Test;

/**
* description:
*
* @author liyazhou
* @create 2017-06-04 19:54
*
* 面试题 49：把字符串转换成整数
*
* 考查点：
*      1. 细节。空字符串，空指针，正负号，前几个数字是0（没啥影响），超出表示范围
*
* 思路：
*      1. 从低位往高位（从右往左）计算
*
*
*/
public class Test49 {

public int strToInt(char[] chars){
String str = String.valueOf(chars);
return Integer.valueOf(str); // Integer.parseInt(str);
}

public int strToInt2(char[] chars){
if (chars == null || chars.length == 0) throw new RuntimeException("Invalid input. chars can be null or length is 0.");

int sign = 1;
int start = 0;
if (chars[0] == '-') {
start = 1;
sign = -1;
}
else if (chars[0] == '+') start = 1;

return strToIntCore(chars, start, sign);
}

private int strToIntCore(char[] chars, int start, int sign) {
// int value = (int)Math.pow(2, 31); 2147483647,已经强转，表示int型最大值

long value = sign;
int base = 1;
for (int i = chars.length - 1; i >= start; i --){
if (chars[i] < '0' || chars[i] > '9')
throw new NumberFormatException(String.valueOf(chars, start, chars.length-start) + ", Invalid input. Element of chars should range from '0' to '9' ");
// try {  intValue += (chars[i] - '0') * base;  }
// catch(Exception except){  throw new RuntimeException("beyond the range of int");  }
value += (chars[i] - '0') * base;
if (value < Integer.MIN_VALUE || value > Integer.MAX_VALUE)
throw new NumberFormatException(String.valueOf(chars, start, chars.length-start) + ", beyond the range of int");
base *= 10;
}
return (int)value;
}

@Test
public void test(){
String[] strs = {
"123",
"-123",
"-0",
"+123",
"+12345671111111110",
"-12345611111111170",
};
for (String str : strs){
System.out.println(strToInt2(str.toCharArray()));
System.out.println(strToInt(str.toCharArray()));
}
// char[] chars = new char[0]; System.out.println(chars.length == 0);
}
}

public int StrToInt(String str){
if (str == null || "".equals(str.trim())) return 0;

int flag = 1;
int i = 0;
if (str.charAt(0) == '-') {
flag = -1;
i = 1;
} else if (str.charAt(0) == '+')
i = 1;

for (; i < str.length(); i ++){
if (str.charAt(i) != '0') break;
}

int num = 0;
int base = 1;
char ch;
for (int j = str.length()-1; j >= i; j --){
ch = str.charAt(j);
if (ch < '0' || ch > '9') return 0;
num += (str.charAt(j)-'0') * base;
base *= 10;
}

return num * flag;

}

public int StrToInt0(String str) {
if (str == null || "".equals(str.trim())) return 0;
int result = 0;
try{
result = Integer.valueOf(str);
}catch (Exception e){
}
return result;
}