问题链接

String to Integer (atoi) | LeetCode OJ

问题描述

写一个方法，将字符串转换为32位有符号的整数。

解决办法

public class Solution {
    public int myAtoi(String str) {
        //if str is empty,return 0
        if (str.length() == 0) {
            return 0;
        }
        //remove empty space
        while (str.length() > 0 && str.charAt(0) == ' ') {
            str = str.substring(1);
        }
        while (str.length() > 0 && str.charAt(str.length() - 1) == ' ') {
            str = str.substring(0, str.length() - 1);
        }
        //if str is empty,return 0
        if (str.length() == 0) {
            return 0;
        }
        //remove plus-minus sign
        boolean isNegative = false;
        if (str.indexOf("-") == 0) {
            isNegative = true;
            str = str.substring(1);
        } else if (str.indexOf("+") == 0) {
            str = str.substring(1);
        }
        //remove zero
        while (str.length() > 0 && str.charAt(0) == '0') {
            str = str.substring(1);
        }
        if (str.length() == 0) {
            return 0;
        }
        
        int result = 0;
        int multiplier = 1;
        int position = 0;
        int[] legitIntArray = new int[str.length()];
        int legitPosition = 0;
        boolean stop = false;
        while (!stop) {
            char c = str.charAt(position);
            try {
                int charNumber = charToInt(c);
                legitIntArray[position] = charNumber;
                position++;
                if (position >= str.length()) {
                    legitPosition = position - 1;
                    stop = true;
                }
            } catch (NumberFormatException e) {
                legitPosition = position - 1;
                stop = true;
            }
        }
        if (legitPosition > 9) {
            if (isNegative) {
                return Integer.MIN_VALUE;
            } else {
                return Integer.MAX_VALUE;
            }
        }
        for (int i = legitPosition; i >= 0; i--) {
            try {
                int charNumber = legitIntArray[i];
                int charNumberWithMultiplier = safeMultiply(charNumber, multiplier);
                result = safeAdd(result, charNumberWithMultiplier);
                multiplier *= 10;
            } catch (ArithmeticException e) {
                if (isNegative) {
                    return Integer.MIN_VALUE;
                } else {
                    return Integer.MAX_VALUE;
                }
            }
        }
        if (isNegative) {
            return -result;
        }
        return result;
    }

    /**
     * @see https://www.securecoding.cert.org/confluence/display/java/NUM00-J.+Detect+or+prevent+integer+overflow
     * @throws ArithmeticException 
     */
    static final int safeAdd(int left, int right) throws ArithmeticException {
        if (right > 0 ? left > Integer.MAX_VALUE - right
                : left < Integer.MIN_VALUE - right) {
            throw new ArithmeticException("Integer overflow");
        }
        return left + right;
    }
    **
     * @see https://www.securecoding.cert.org/confluence/display/java/NUM00-J.+Detect+or+prevent+integer+overflow
     * @throws ArithmeticException 
     */
    static final int safeMultiply(int left, int right) throws ArithmeticException {
        if (right > 0 ? left > Integer.MAX_VALUE / right
                || left < Integer.MIN_VALUE / right
                : (right < -1 ? left > Integer.MIN_VALUE / right
                        || left < Integer.MAX_VALUE / right
                        : right == -1
                        && left == Integer.MIN_VALUE)) {
            throw new ArithmeticException("Integer overflow");
        }
        return left * right;
    }

    public static int charToInt(char c) throws NumberFormatException {
        switch (c) {
            case '0':
                return 0;
            case '1':
                return 1;
            case '2':
                return 2;
            case '3':
                return 3;
            case '4':
                return 4;
            case '5':
                return 5;
            case '6':
                return 6;
            case '7':
                return 7;
            case '8':
                return 8;
            case '9':
                return 9;
        }

        throw new NumberFormatException("Illegal char " + String.valueOf(c));
    }
}