算法基础（一）---- 不使用Integer.parseInt将String转为Int

题目：不使用Integer.parseInt(str)方法，将String类型的整数字符串转为Int类型的整数

例如将"-46570"转为-46570，将"146570"转为146570，注意，也不能使用Integer.valueOf(str).intValue();

算法一：

public class test2 {
	public static void main(String args[]){
		String str1 = "-46570";
		System.out.println(trans(str1));
		String str2 = "146570";
		System.out.println(trans(str2));
	}
	public static int trans(String target){
		String resultStr = target;
		boolean flag = true;
		if (target.charAt(0)=='-') {
			resultStr = target.substring(1, target.length());
			flag = false;
		}
		int result = 0;
		for (int i = resultStr.length()-1,j=0; i>=0 ; i--,j++) {
			result += (resultStr.charAt(i)-48)*(int)Math.pow(10, j);
		}//直接将单个字符转为数字
		if (flag) {
			return result;
		}
		else {
			return -result;
		}
	}
}

算法二：

public class test2 {
	public static void main(String args[]){
		String str1 = "-46570";
		System.out.println(trans(str1));
		String str2 = "146570";
		System.out.println(trans(str2));
	}
	public static int trans(String target){
		String resultStr = target;
		boolean flag = true;
		if (target.charAt(0)=='-') {
			resultStr = target.substring(1, target.length());
			flag = false;
		}
		int result = 0;
		Map<String, Integer> map = new HashMap<String, Integer>();
		for (int i = 0; i < 10; i++) {
			map.put(String.valueOf(i), i);
		}
		for (int i = resultStr.length()-1,j=0; i>=0 ; i--,j++) {
			result += map.get(String.valueOf(resultStr.charAt(i)))*(int)Math.pow(10, j);
		}//使用map查找单个字符串对应的数字
		if (flag) {
			return result;
		}
		else {
			return -result;
		}
	}
}

可查看Integer.parseInt源码方法是怎么做的

    public static int parseInt(String s) throws NumberFormatException {
        return parseInt(s,10);
    }

public static int parseInt(String s, int radix) throws NumberFormatException {
	if (s == null) {
		throw new NumberFormatException("null");
	}
	if (radix < Character.MIN_RADIX) {
		throw new NumberFormatException("radix " + radix + " less than Character.MIN_RADIX");
	}
	if (radix > Character.MAX_RADIX) {
		throw new NumberFormatException("radix " + radix + " greater than Character.MAX_RADIX");
	}
	int result = 0;
	boolean negative = false;
	int i = 0, max = s.length();
	int limit;
	int multmin;
	int digit;
	if (max > 0) {
		if (s.charAt(0) == '-') {
			negative = true;
			limit = Integer.MIN_VALUE;
			i++;
		} else {
			limit = -Integer.MAX_VALUE;
		}
		multmin = limit / radix;
		if (i < max) {
			digit = Character.digit(s.charAt(i++), radix);
			if (digit < 0) {
				throw NumberFormatException.forInputString(s);
			} else {
				result = -digit;
			}
		}
		while (i < max) {
			// Accumulating negatively avoids surprises near MAX_VALUE
			digit = Character.digit(s.charAt(i++), radix);
			if (digit < 0) {
				throw NumberFormatException.forInputString(s);
			}
			if (result < multmin) {
				throw NumberFormatException.forInputString(s);
			}
			result *= radix;
			if (result < limit + digit) {
				throw NumberFormatException.forInputString(s);
			}
			result -= digit;
		}
	} else {
		throw NumberFormatException.forInputString(s);
	}
	if (negative) {
		if (i > 1) {
			return result;
		} else { /* Only got "-" */
			throw NumberFormatException.forInputString(s);
		}
	} else {
		return -result;
	}
}