008.String To Integer

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Solution 1: accepted

A primitive solution that manipulates Strings. Spent 7 ms and beats 9.5%.

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public static int myAtoi(String str) {
	str = str.trim();
	if (str.length() == 0)
		return 0;
	if ((str.charAt(0) < '0' || str.charAt(0) > '9') && str.charAt(0) != '-' && str.charAt(0) != '+') {
		return 0;
	}
	String sign = "";
	int index = 0;
	if (str.charAt(0) == '-' || str.charAt(0) == '+') {
		index = 1;
		if (str.length() == 1 || str.charAt(1) < '0' || str.charAt(1) > '9')
			return 0;
		if (str.charAt(0) == '-')
			sign = "-";
	}
	StringBuilder sb = new StringBuilder();
	for (int i = index; i < str.length(); i++) {
		if (str.charAt(i) >= 48 && str.charAt(i) <= 57) {
			sb.append(str.charAt(i));
		} else
			break;
	}
	String value = sb.toString();
	if (value.length() >= 10 && sign.equals("-")) {
		if (value.length() >= 11)
			return Integer.parseInt("-2147483648");
		else if (value.length() == 10 && value.compareTo("2147483648") > 0)
			return Integer.parseInt("-2147483648");
		else
			return Integer.parseInt(sign + value);
	}
	else if (value.length() >= 10 && sign.equals("")) {
		if (value.length() >= 11)
			return Integer.parseInt("2147483647");
		else if (value.length() == 10 && value.compareTo("2147483647") > 0)
			return Integer.parseInt("2147483647");
		else
			return Integer.parseInt(value);
	} else
		return Integer.parseInt(sign + value);
}

Solution 2: accepted

Improved based on the first one, it now beats 14.9%.

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public static int myAtoi(String str) {
	int max = Integer.MAX_VALUE;
	int min = Integer.MIN_VALUE;
	int sign = 1;
	boolean start = false;
	StringBuilder sb = new StringBuilder();
	for (int i = 0; i < str.length(); i++) {
		char thisChar = str.charAt(i);
		if (thisChar == ' ' && !start) {
			continue;
		} else if ((thisChar == '-' || thisChar == '+') && !start) {
			start = true;
			if (thisChar == '-')
				sign = -1;
		} else if (thisChar < '0' || thisChar > '9')
			break;
		else {
			start = true;
			sb.append(thisChar);
		}
	}
	if (sb.length() == 0)
		return 0;
	String value = sb.toString();
	if (value.length() >= 10 && sign == -1) {
		if (value.length() >= 11)
			return min;
		else if (value.length() == 10 && value.compareTo("2147483648") >= 0)
			return min;
		else
			return Integer.parseInt(value) * sign;
	}
	else if (value.length() >= 10 && sign == 1) {
		if (value.length() >= 11)
			return max;
		else if (value.length() == 10 && value.compareTo("2147483647") >= 0)
			return max;
		else
			return Integer.parseInt(value);
	} else
		return Integer.parseInt(value) * sign;
}

Solution 3: accepted

Manipulate numbers are more efficient than manipulating Strings. Now we have 85%.

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public static int myAtoi(String str) {
	int max = Integer.MAX_VALUE;
	int min = Integer.MIN_VALUE;
	int digit; 
	int result = 0; 
	boolean isNegative = false;
	boolean start = false; //whether we have the first valid character 
	for (int i = 0; i < str.length(); i++) {
		char thisChar = str.charAt(i);
		if (!start && thisChar == ' ') {
			continue;
		} else if (!start && (thisChar == '-' || thisChar == '+')) {
			start = true;
			if (thisChar == '-')
				isNegative = true;
		} else if (thisChar < '0' || thisChar > '9')
			break;
		else {
			start = true;
			digit = thisChar - '0';
			if (result > max / 10 || (result == max / 10 && digit > 7)) { // 7 is max % 10
				if (isNegative)
					return min;
				else
					return max;
			}
			result = result * 10 + digit;
		}
	}