题目：
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).
For example,
S = “ADOBECODEBANC”
T = “ABC”
Minimum window is “BANC”.
Note:
If there is no such window in S that covers all characters in T, return the emtpy string “”.
If there are multiple such windows, you are guaranteed that there will always be only one unique minimum window in S.
题意就是在S中选一个最小的子串，使得包含T中所有的字母。
思路：
i,j表示S串的起点与终点；
一个数组保存 int count[255] T串在s中的字母出现的次数；
一个队列保存字母出现的顺序；
minValue保存最小的长度；
num 表示在i,j 之间的有效个数；
思路：滑动i,j直到num =s的长度，然后根据队列中的值去移动i，和更新count ,num ，minvalue
Refer from: http://blog.csdn.net/pi9nc/article/details/12112373