把字符串转换成整数

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题目67：把字符串转换成整数

实现一个函数stringToInt,实现把字符串转换成整数这个功能，不能使用atoi或者其他类似的库函数。

分析

该题目主要考察的就是边界条件：

  • 空字符串
    if (num == null || num.length() < 1)
  • 在数字范围
    c >= '0' && c <= '9'
  • 数据上下溢出
    if (tmp > 0x8000_0000L)
  • 只有正负号
  • 有无正负号
    if (first == '-')
    else if (first == '+')
  • 错误标志输出
    throw new NumberFormatException(num)

思路

代码实现

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public class _67 {
	public static int stringToInt(String num) {
		if (num == null || num.length() < 1) {
			throw new NumberFormatException(num);
		}
		char first = num.charAt(0);
		if (first == '-') {
			return parseString(num, 1, false);
		} else if (first == '+') {
			return parseString(num, 1, true);
		} else if (first <= '9' && first >= '0') {
			return parseString(num, 0, true);
		} else {
			throw new NumberFormatException(num);
		}
	}

	private static boolean isDigit(char c) {
		return c >= '0' && c <= '9';
	}

	private static int parseString(String num, int index, boolean positive) {
		if (index >= num.length()) {//index    开始解析的索引位置
			throw new NumberFormatException(num);
		}

		int result;
		long tmp = 0;
		while (index < num.length() && isDigit(num.charAt(index))) {
			tmp = tmp * 10 + num.charAt(index) - '0';//转换的关键步骤
			// 保证求的得的值不超出整数的最大绝对值
			if (tmp > 0x8000_0000L) {
				throw new NumberFormatException(num);
			}
			index++;
		}
		if (positive) {//正数
			if (tmp >= 0x8000_0000L) {
				throw new NumberFormatException(num);
			} else {
				result = (int) tmp;
			}
		} else {//负号开头
			if (tmp == 0x8000_0000L) {
				result = 0x8000_0000;
			} else {
				result = (int) -tmp;
			}
		}//最后的长度判定，避免 1a123
		if(sizeOfInt(result)+index<num.length()) {
			throw new NumberFormatException(num);
		}
		return result;
	}
	
	//判断一个int值是几位数
	final static int[] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,
			99999999, 999999999, Integer.MAX_VALUE };
	static int sizeOfInt(int x) {
		for (int i = 0; ; i++) {//从一位数开始对比，如果比一位数大便继续对比二位数
			if (x <= sizeTable[i]) {
				return i + 1;
			}
		}
	}

	public static void main(String[] args) {
		System.out.println("Integer.MIN_VALUE is"+Integer.parseInt(Integer.MIN_VALUE + ""));// -2147483648
		System.out.println("Integer.MAX_VALUE is "+Integer.parseInt(Integer.MAX_VALUE + ""));//2147483647
		System.out.println("123 -> "+stringToInt("123"));// 123
		System.out.println("+123 -> "+stringToInt("+123"));// 123
		System.out.println("-123 -> "+stringToInt("-123"));// -123
		System.out.println("+2147483647 -> "+stringToInt("+2147483647"));// 2147483647
		System.out.println("-2147483647 -> "+stringToInt("-2147483647"));// -2147483647
		System.out.println("-2147483648 -> "+stringToInt("-2147483648"));// -2147483648
//		 System.out.println("1a123 -> "+stringToInt("1a123"));//java.lang.NumberFormatException: 1a123
//		 System.out.println("+2147483649 -> "+stringToInt("+2147483649"));//java.lang.NumberFormatException:+2147483649
//		 System.out.println("-2147483649 -> "+stringToInt("-2147483649"));//java.lang.NumberFormatException:-2147483649
//		 System.out.println("空字符-> "+stringToInt(""));//java.lang.NumberFormatException
//		 System.out.println("+2147483648 -> "+stringToInt("+2147483648"));//java.lang.NumberFormatException:+2147483648
//		 System.out.println("+ ->"+stringToInt("+"));//java.lang.NumberFormatException: +
//		 System.out.println("- ->"+stringToInt("-"));//java.lang.NumberFormatException: -
	}
}

输出：（抛异常的输出见注释）

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Integer.MIN_VALUE is-2147483648
Integer.MAX_VALUE is 2147483647
123 -> 123
+123 -> 123
-123 -> -123
+2147483647 -> 2147483647
-2147483647 -> -2147483647
-2147483648 -> -2147483648

一个机灵：

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(res << 1) + (res << 3) 即 res*2+res*8=res*10
> >