LeetCode——String to Integer (atoi)

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

If no valid conversion could be performed, a zero value is returned. If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.

实现 atoi 将字符串转为整数。

需要考虑正负号，空格，非数字字符出现的话，其后部分随之忽略。

提交了如下的代码，结果运行超时：

	public static int atoi(String str) {
		str = str.trim();
		if(str.isEmpty())
			return 0;
		char[] ch = str.toCharArray();
		for(int i=1;i<ch.length;i++){
			if(!Character.isDigit(ch[i])){
				str = str.substring(0, i);
				break;
			}
		}
		ch = str.toCharArray();
		int ret = 0;
		for (int i = 0; i < ch.length; i++) {
			if (ch[0] == '-') {
				ret = ret * 10 - (ch[i + 1] - 48);
				if (i == ch.length - 2)
					break;
			} else if (ch[0] == '+') {
				ret = ret * 10 + (ch[i + 1] - 48);
				if (i == ch.length - 2)
					break;
			} else {
				ret = ret * 10 + (ch[i] - 48);
			}
		}
		return ret;
	}

	public static int atoi(String str) {
		int max = Integer.MAX_VALUE;
		int min = -Integer.MIN_VALUE;
		long result = 0;
		str = str.trim();
		int len = str.length();
		if (len < 1)
			return 0;
		int start = 0;
		boolean neg = false;

		if (str.charAt(start) == '-' || str.charAt(start) == '+') {
			if (str.charAt(start) == '-')
				neg = true;
			start++;
		}

		for (int i = start; i < len; i++) {
			char ch = str.charAt(i);

			if (ch < '0' || ch > '9')
				break;
			result = 10 * result + (ch - '0');
			if (!neg && result > max)
				return max;
			if (neg && -result < min)
				return min;

		}
		if (neg)
			result = -result;

		return (int) result;
	}