8. String to Integer (atoi)

class Solution {
    public int myAtoi(String str) {
        int len = str.length();
        if(len==0){
            return 0;
        }
        long result=0;
        int i = 0;
        int scount = 0;
        boolean negative = false;
        while(str.charAt(i)==' '&&i<len-1){
            i++;
            scount++;
        }
        if(str.charAt(i)=='-'){
            if(len==scount+1)
                return 0;
            negative = !negative;
            i++;
        }else if(str.charAt(i)=='+'){
            if(len==scount+1)
                return 0;
            i++;
        }
        while(str.charAt(i)=='0'&&i<len-1){
            i++;
        }

        for(;i<len;i++){
            if(str.charAt(i)>='0'&&str.charAt(i)<='9'){
                result*=10;
                result += Character.digit(str.charAt(i),10);
                if(result>(long)Integer.MAX_VALUE+1||(result==(long)Integer.MAX_VALUE+1&&!negative)){
                    return negative?Integer.MIN_VALUE:Integer.MAX_VALUE;
                }
            }else{
                break;
            }
        }
        return negative?-(int)result:(int)result;
    }
}

看一下java.lang.Integer的实现(并不是本题的解)

public static int parseInt(String s, int radix)
                throws NumberFormatException
    {
        /*
         * WARNING: This method may be invoked early during VM initialization
         * before IntegerCache is initialized. Care must be taken to not use
         * the valueOf method.
         */

        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, len = s.length();
        int limit = -Integer.MAX_VALUE;
        int multmin;
        int digit;

        if (len > 0) {
            char firstChar = s.charAt(0);
            if (firstChar < '0') { // Possible leading "+" or "-"
                if (firstChar == '-') {
                    negative = true;
                    limit = Integer.MIN_VALUE;
                } else if (firstChar != '+')
                    throw NumberFormatException.forInputString(s);

                if (len == 1) // Cannot have lone "+" or "-"
                    throw NumberFormatException.forInputString(s);
                i++;
            }
            multmin = limit / radix;
            while (i < len) {
                // 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);
        }
        return negative ? result : -result;
    }