LintCode: String to Integer

// https://code.dennyzhang.com/string-to-integer
// Basic Ideas:
// Complexity: Time O(1), Space O(1)
/**
 * @param str: A string
 * @return: An integer
 */
func stringToInteger (str string) int {
    res := 0
    negative := false
    for i, ch := range str {
        if i == 0 && ch == '-' {
            negative = true
            continue
        }
        res = res*10 + int(ch-'0')
    }
    if negative { res = -res }
    return res
}