for循环和if-else语句中的代码

Question

I was trying to solve this problem: https://leetcode.com/problems/longest-substring-without-repeating-characters/

The following code passed all tests in 44 ms.

    for (int i = 0; i < s.length(); i++){
        if (!mp.containsKey(s.charAt(i))){
            mp.put(s.charAt(i), i);
            //max = Math.max(max, i-first+1);
        }
        else {
            if (mp.get(s.charAt(i))+1>=first){
                first = mp.get(s.charAt(i))+1;
            }
            mp.put(s.charAt(i), i);
            //max = Math.max(max, i-first+1);
        }
        max = Math.max(max, i-first+1);
    }

But the following code passed all tests in only 20 ms.

    for (int i = 0; i < s.length(); i++){
        if (!mp.containsKey(s.charAt(i))){
            mp.put(s.charAt(i), i);
            max = Math.max(max, i-first+1);
        }
        else {
            if (mp.get(s.charAt(i))+1>=first){
                first = mp.get(s.charAt(i))+1;
            }
            mp.put(s.charAt(i), i);
            max = Math.max(max, i-first+1);
        }
    }

Why is there such a significant difference? The max value is changed only once in both of the samples but changing it in the if-else statements is far more efficient than changing it at the end of the for loop. Is this an exception or should we follow this standard whenever coding?

Answer 1

On simplifying (not early optimizing, see max!) one gets

for (int i = 0; i < s.length(); i++) {
    char ch = s.charAt(i);
    Integer n = mp.get(ch):
    if (n != null) {
        first = Math.max(first, n + 1);
    }
    mp.put(ch, i);
    max = Math.max(max, i - first + 1);
}

Remark the double put of value i in the original version. If the first Math.max is replaced with an if, the code might be faster.

It is hard to make a statement wrt to speed here for the two original versions, maybe the hotspot compiler saw the redundancy. Or whatever.

It would be nice seeing a java 8 version, using mp.putIfAbsent or such, but that might meke it slower.