題目：實(shí)現(xiàn)一個(gè)函數(shù)stringToInt,實(shí)現(xiàn)把字符串轉(zhuǎn)換成整數(shù)這個(gè)功能鼎天，不能使用atoi或者其他類似的庫(kù)函數(shù)。

代碼如下：

package demo;

/**
 * 把字符串轉(zhuǎn)換成整數(shù)
 * 
 * @author xiangdonglee
 *
 */
public class Test49 {
    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);
        }
    }

    /**
     * 對(duì)字符串進(jìn)行解析
     * 
     * @param num
     *            數(shù)字串
     * @param index
     *            開(kāi)始解析的索引
     * @param positive
     *            是正數(shù)還是負(fù)數(shù)
     * @return 返回結(jié)果
     */
    private static int parseString(String num, int index, boolean positive) {
        if (index >= num.length()) {
            throw new NumberFormatException(num);
        }
        int result;
        long tmp = 0;
        while (index < num.length() && isDigit(num.charAt(index))) {
            tmp = tmp * 10 + num.charAt(index) - '0';
            // 保證求得的值不超過(guò)整數(shù)的最大絕對(duì)值
            if (tmp > 0x8000_0000L) {
                throw new NumberFormatException(num);
            }
            index++;
        }
        if (positive) {
            if (tmp >= 0x8000_0000L) {
                throw new NumberFormatException(num);
            } else {
                result = (int) tmp;
            }
        } else {
            if (tmp == 0x8000_0000L) {
                result = 0x8000_0000;
            } else {
                result = (int) -tmp;
            }
        }
        return result;
    }

    /**
     * 判斷字符是否是數(shù)字
     * 
     * @param c
     *            字符
     * @return true：是暑竟；false：否
     */
    private static boolean isDigit(char c) {
        return c >= '0' && c <= '9';
    }

    public static void main(String[] args) {
        System.out.println(Integer.parseInt(Integer.MIN_VALUE + ""));
        System.out.println(0x8000_0000L);
        // System.out.println(stringToInt(""));
        System.out.println(stringToInt("123"));
        System.out.println(stringToInt("+123"));
        System.out.println(stringToInt("-123"));
        System.out.println(stringToInt("1a123"));
        System.out.println(stringToInt("+2147483647"));
        System.out.println(stringToInt("-2147483647"));
        // System.out.println(stringToInt("+2147483648"));
        // System.out.println(stringToInt("-2147483648"));
        // System.out.println(stringToInt("+"));
        // System.out.println(stringToInt("-"));
    }
}

運(yùn)行結(jié)果

來(lái)源：http://blog.csdn.net/derrantcm/article/details/46811799