LeetCode 8: String to Integer (atoi)

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本文介绍了一个将字符串转换为整数（atoi）的算法实现。该方法首先跳过字符串中的空白字符，然后读取首个非空白字符开始的数字序列，并将其解释为数值。文章详细解释了处理正负号、非数字字符以及溢出情况的方法。

Implement atoi which converts a string to an integer.

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.

Note:

Only the space character ’ ’ is considered as whitespace character.
Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.

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.
             Thefore INT_MIN (−231) is returned.

my solution:

class Solution {
    public Solution() {
        init();
    }
    static public Map<Character,Integer> c2n = null;
	static public void init() {
		c2n = new HashMap<>();
    	c2n.put('0', 0);
    	c2n.put('1', 1);
    	c2n.put('2', 2);
    	c2n.put('3', 3);
    	c2n.put('4', 4);
    	c2n.put('5', 5);
    	c2n.put('6', 6);
    	c2n.put('7', 7);
    	c2n.put('8', 8);
    	c2n.put('9', 9);
	}
	static public boolean ifValidNum(char c) {
		return c2n.containsKey(c);
	}
	static public boolean ifValidBegin(char c) {
		return c2n.containsKey(c) || c=='+' || c=='-' || c==' ' ;
	}
    static public int myAtoi(String str) {
    	//init();
    	int result = 0;
    	boolean flag = false;
    	boolean positive = true;
    	boolean overflow = false;
    	for(int i = 0; i < str.length();++i) {
    		char c = str.charAt(i);
    		if(!flag ) {
    			if(!ifValidBegin(c)){
	    			break;
    			}
    			if( c!=' ' ) {
    				flag = true;
        			if(ifValidNum(c)) {
        				result = c2n.get(c);
        			}else {
        				if(c == '+')
        					positive = true;
        				else 
        					positive = false;
        			}
    			}

    		}else {
    			if(!ifValidNum(c)) {
    				break;
    			}else {
    				if(Math.abs(result) > 0x7fffffff/10) {
    					overflow = true;
    					break;
    				}else if(Math.abs(result) == 0x7fffffff/10){
                        if((positive && c2n.get(c)>Integer.MAX_VALUE%10) ||
                          (!positive && c2n.get(c)>-(Integer.MIN_VALUE%10))){
                            overflow = true;
    					    break;
                        }      
                    }
    				result = positive?result*10+c2n.get(c):result*10-c2n.get(c);
    			}
    		} 		
    	}
    	if(overflow)
    		return positive?Integer.MAX_VALUE:Integer.MIN_VALUE;

        return result;
    }
}