最小覆盖子串

给你一个字符串 S、一个字符串 T 。请你设计一种算法，可以在 O(n) 的时间复杂度内，从字符串 S 里面找出：包含 T 所有字符的最小子串。

class Solution(object):
    def minWindow(self, s, t):
        need = {}
        for c in t:
            need[c] = need.get(c, 0) + 1
        needLen = len(t)
        i = 0
        res = (0, 999999999999)
        for j, c in enumerate(s):
            if need.get(c, 0) > 0:
                needLen -= 1
            need[c] = need.get(c, 0) - 1
            if needLen == 0:
                while True:
                    if need.get(s[i], 0) == 0:
                        break
                    else:
                    	need[s[i]] = need.get(s[i], 0) + 1
                        i += 1
                if j - i < res[1] - res[0]:
                    res = (i, j)
                    print(need)
                needLen += 1
                i += 1
                need[s[i-1]] = need.get(s[i-1], 0) + 1
        return ('' if res[1] > len(s) else s[res[0]:res[1]+1])