76. Minimum Window Substring

Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).

Example:

Input: S = "ADOBECODEBANC", T = "ABC"
Output: "BANC"

Note:

• If there is no such window in S that covers all characters in T, return the empty string "".
• If there is such window, you are guaranteed that there will always be only one unique minimum window in S.

Code:

class Solution(object):
    def minWindow(self, s, t):
        """
        :type s: str
        :type t: str
        :rtype: str
        """
        need = collections.Counter(t)    #hash table to store char frequency
        missing = len(t)                 #total number of chars we care
        start, end = 0, 0
        i = 0
        for j, char in enumerate(s, 1):    #index j from 1
            if need[char] > 0:
                missing -= 1
            need[char] -= 1
            if missing == 0:                     #match all chars
                while i < j and need[s[i]] < 0:  #remove chars to find the real start
                    need[s[i]] += 1
                    i += 1
                need[s[i]] += 1  #make sure the first appearing char satisfies need[char]>0
                missing += 1     #we missed this first char, so add missing by 1
                if end == 0 or j-i < end-start:  #update window
                    start, end = i, j
                i += 1                           #update i to start+1 for next window
        return s[start:end]