leetcode: 8. String to Integer (atoi)

Difficulty

Medium

Description

/**
 * Example 1:
 * Input: "42"
 * Output: 42
 * 
 * Example 2:
 * 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:
 * Input: "4193 with words"
 * Output: 4193
 * Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
 * 
 * Example 4:
 * 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:
 * Input: "-91283472332"
 * Output: -2147483648
 * Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
 *              Therefore INT_MIN (−231) is returned.
 *
 */

Solution

class Solution {
    public int myAtoi(String str) {
	  	int i = 0;
		int ret = 0;
		int sign= 1;
		int digit;
		if (str == null)
			return 0;
		str = str.trim();
		if (str.length() == 0)
			return 0;
		if (str.charAt(0) == '+') {sign = 1; i++;}
		if (str.charAt(0) == '-') {sign = -1; i++;}
		while (str.length() > i) {
	    	if (!Character.isDigit(str.charAt(i)))
		    	return (int)ret*sign;
		    digit = str.charAt(i) - '0';
		    if (ret > (Integer.MAX_VALUE - digit) / 10 || (ret == Integer.MAX_VALUE / 10 && digit > 7)) 
		    	return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;  		    //检查是否越界
			ret = ret * 10 + digit;
			i++;
		}
		return (int)ret*sign;  
    }
}