Leetcode: Substring with Concatenation of All Words

Description:

You are given a string, S, and a list of words, L, that are all of the same length. Find all starting indices of substring(s) in S that is a concatenation of each word in L exactly once and without any intervening characters.

For example, given:
S: "barfoothefoobarman"
L: ["foo", "bar"]

You should return the indices: [0,9].
(order does not matter).

分析： 这道题目告诉我们一定要认真读题目！ 题目做了限制，会使得解题容易的多： 单词表中的words长度一样！ 

这样题目只要建立了哈希表，即可使用循环解决。 注意中间也有一些简单的trick,比如长度限制扫描范围等。

class Solution {
public:
    vector<int> findSubstring(string S, vector<string> &L) {
        vector<int> result;
        int nums = L.size();
        if(nums==0) return result;
        int wordsz = L[0].size();
        if(S.size()<(wordsz*nums)) return result;
        
        unordered_map<string,int> dict,backup;
        string target;
        for(int i=0;i<nums;i++)
            dict[L[i]]++;
        backup =dict;
        
        for(int i=0;i<=S.size()-(wordsz*nums);i++)
        {
            int hitnum = 0;
            for(int j =i;j!=i+wordsz*nums;j+=wordsz)
            {
                target.assign(S.begin()+j,S.begin()+j+wordsz);
                auto p = dict.find(target);
                if(p!=dict.end() && p->second)
                {
                    hitnum++;
                    p->second--;
                }
                else{
                    dict = backup;
                    break;
                }
                if(hitnum == nums)
                {
                    dict = backup;
                    result.push_back(i);
                    break;
                }
            }
            
        }
        return result;
    }
};