Permutation in String

// Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise.
// In other words, return true if one of s1's permutations is the substring of s2.

// Eg: s1 = 'ab', s2 = 'eidbaooo'
//     Ans = True

class Solution:
    def checkInclusion(self, s1: str, s2: str) -> bool:
        
        net = dict()
        for c in s1:
            net[c] = net.get(c, 0) + 1
        
        wanted = 0
        i = 0
        for j in range(len(s2)):
            if s2[j] in net:
                net[s2[j]] -= 1
                if net[s2[j]] == 0:
                    wanted += 1
            
            if wanted == len(net):
                return True         // s1 exists in s2
            
            if (j - i + 1) == len(s1):
                if s2[i] in net:
                    if net[s2[i]] == 0:
                        wanted -= 1
                    net[s2[i]] += 1
                i += 1
        
        return False