原题链接: https://oj.leetcode.com/problems/string-to-integer-atoi/

这道题目着重点是 corner case 的处理，一串字符串在转换为整数时，需要考虑它的正负号，以及是否溢出。

public int atoi(String str) {
    long resultNum = 0; 

    if (str == null || str.equals("")) {
        return (int) resultNum;
    } 

    str = str.trim();
    boolean negative = false;
    char c;

    for (int i = 0, length = str.length(); i < length; i++) {
        c = str.charAt(i);

        if (i == 0 && (c == '-' || c == '+')) {
            if (c == '-') {
                negative = true;                
            }
        } else if ('0' <= c && c <= '9') {
            resultNum = resultNum * 10 + (c - '0');

            if (resultNum > Integer.MAX_VALUE && !negative) {
                resultNum = Integer.MAX_VALUE;
            } else if (negative && resultNum * -1 < Integer.MIN_VALUE) {
                resultNum = Integer.MIN_VALUE;
            }
        } else {
            break;
        }
    }

    return (int) (negative == true ? resultNum * -1 : resultNum);           
}