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String to Integer

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问题描述

Example 1:

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Input: "42"
Output: 42

Example 2:

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Input: "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
             Then take as many numerical digits as possible, which gets 42.

Example 3:

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Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.

Example 4:

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Input: "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical 
             digit or a +/- sign. Therefore no valid conversion could be performed.

Example 5:

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Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
             Thefore INT_MIN (−231) is returned.

解决方案

主要就是要考虑的细节比较多，特殊的情况及其处理在注释中给出

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int myAtoi(string str)
{
	int len = str.length();
	int s = 0;
	long long res = 0;
	int numLen = 0;
	bool minus = false;
	bool hasNum = false;
	while (s < len && ((str[s] == ' ' && !hasNum) || str[s] == '0'))
	{
		//第二个条件针对"0  123"
		if (str[s] == '0')hasNum = true;
		++s;
	}
	if (!hasNum)
	{//hasNum的判断针对"0-1"
		//if-else语句针对"-+1"，"-+1"，"+1"
		if (str[s] == '-') { ++s; minus = true; }
		else if (str[s] == '+') { ++s; }
	}
	while (s < len && (str[s] == '0'))++s;//针对"   +0 123"，"-000000000000001"，"  0000000000012345678"
	if (str[s]<'0' || str[s]>'9')return 0;
	while (s < len&&str[s] >= '0' && str[s] <= '9'&&numLen<11)
	{//INT_MAX是10位数，numLen针对"20000000000000000000"
		res *= 10;
		res += (str[s] - '0');
		++s; ++numLen;
	}
	if (minus)res *= (-1);
	if (res > INT_MAX)return INT_MAX;
	if (res < INT_MIN) return INT_MIN;
	return res;
}

另外看到一个解决方法，在处理数字串前面的空白、0、正负号时更简洁：

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class Solution {
public:
    int myAtoi(string str) {
        bool neg = false, caled = false;
        int res = 0;
        int tmp;
        for(string::iterator it=str.begin(); it!=str.end(); ++it) {
            if ( *it == ' ' && !caled) continue;
            if ( *it == '-' && !caled) { neg = true; caled = true; continue; }
            if ( *it == '+' && !caled) { neg = false; caled = true; continue; }
            if ( *it >= '0' && *it <= '9') {
                tmp = *it - '0';
                caled = true;
                if ( res > 0 && !neg &&
                    (( res > INT_MAX / 10) || ((INT_MAX - res*10) < tmp)) ) {
                    res = INT_MAX;
                    return res;}
                if (res > 0 && neg &&
                    (( -1 * res < INT_MIN / 10) || (res*-10 <= INT_MIN + tmp)) ) {
                    res = INT_MIN;
                    return res;}
                res = res*10 + tmp;
                continue;
            }
            break;
        }
        if (caled) res = neg ? -1 * res : res;
        return res;
    }
};