Minimum Window Substring

class Solution {
public:
    string minWindow(string s, string t) {
        if(s.empty()) return "";
        if(s.size() < t.size()) return "";

        //hash table
        const int ASCII_MAX = 256;
        int appear_count[ASCII_MAX];
        int expect_count[ASCII_MAX];
        fill(appear_count, appear_count + ASCII_MAX, 0);
        fill(expect_count, expect_count + ASCII_MAX, 0);
        //initialize
        int minWidth = INT_MAX, min_start = 0;//window's size, starting point
        int wnd_start = 0;
        int appeared = 0;
        for(int i = 0; i < t.size(); ++i) {
            expect_count[t[i]]++;
        }

        for(int end = 0; end < s.size(); ++end) {
            if(expect_count[s[end]] > 0) {
                appear_count[s[end]]++;
                if(appear_count[s[end]] <= expect_count[s[end]]) {
                    appeared++;
                }
            }
            if(appeared == t.size()) {//find a complete t in s
                while(appear_count[s[wnd_start]] > expect_count[s[wnd_start]] || expect_count[s[wnd_start]] == 0) {
                    appear_count[s[wnd_start]]--;
                    wnd_start++;
                }
                if(minWidth > end - wnd_start + 1) {
                    minWidth = end - wnd_start + 1;
                    min_start = wnd_start;
                }
            }
        }

        if(minWidth == INT_MAX) return "";
        else return s.substr(min_start, minWidth);
    }
};