LeetCode – Substring with Concatenation of All Words (Java)

Category: Algorithms >> Interview June 13, 2014

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

For example, given: s="barfoothefoobarman" & words=["foo", "bar"], return [0,9].

Analysis

This problem is similar (almost the same) to Longest Substring Which Contains 2 Unique Characters.

Since each word in the dictionary has the same length, each of them can be treated as a single character.

Java Solution

public List<Integer> findSubstring(String s, String[] words) {
    ArrayList<Integer> result = new ArrayList<Integer>();
    if(s==null||s.length()==0||words==null||words.length==0){
        return result;
    } 
 
    //frequency of words
    HashMap<String, Integer> map = new HashMap<String, Integer>();
    for(String w: words){
        if(map.containsKey(w)){
            map.put(w, map.get(w)+1);
        }else{
            map.put(w, 1);
        }
    }
 
    int len = words[0].length();
 
    for(int j=0; j<len; j++){
        HashMap<String, Integer> currentMap = new HashMap<String, Integer>();
        int start = j;//start index of start
        int count = 0;//count totoal qualified words so far
 
        for(int i=j; i<=s.length()-len; i=i+len){
            String sub = s.substring(i, i+len);
            if(map.containsKey(sub)){
                //set frequency in current map
                if(currentMap.containsKey(sub)){
                    currentMap.put(sub, currentMap.get(sub)+1);
                }else{
                    currentMap.put(sub, 1);
                }
 
                count++;
 
                while(currentMap.get(sub)>map.get(sub)){
                    String left = s.substring(start, start+len);
                    currentMap.put(left, currentMap.get(left)-1);
 
                    count--;
                    start = start + len;
                }
 
 
                if(count==words.length){
                    result.add(start); //add to result
 
                    //shift right and reset currentMap, count & start point         
                    String left = s.substring(start, start+len);
                    currentMap.put(left, currentMap.get(left)-1);
                    count--;
                    start = start + len;
                }
            }else{
                currentMap.clear();
                start = i+len;
                count = 0;
            }
        }
    }
 
    return result;
}

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Derek

why is the first loop from 0 to length of the longest word.

for(int j=0; j<len; j++)

Why isn't this 0 to the length of the input string s? I'm guessing that you want to find all possibilities with the search starting at each index of the input string. well 0 to the length of the input string

for(int j=0; j<s.length()-len; j++)
for(int i=j; i<=s.length()-len; i=i+len)

Thanks,
Derek
Ahmad El-Qaoud

I solved by a better solution which happens to be more clear and has a better performance using the indexOf method and comparing the indices as follows:

public static List searchIndecies(String s, String arr[]) {

if (s == null || s.length() == 0 || arr == null || arr.length == 0) {
return null;
}
int wordLength = arr[0].length();
List returnedIndecies = new ArrayList();
List indecies = new ArrayList();
for (int i = 0; i < s.length(); i++) {
indecies.clear();
for (int m = 0; m < arr.length; m++) {
int index = s.indexOf(arr[m], i);
if (index != -1) {
indecies.add(index);
}
}
Collections.sort(indecies);
boolean validSeq = false;
for (int n = 1; n < indecies.size() && indecies.size() == arr.length; n++) {
if (indecies.get(n – 1) != indecies.get(n) – wordLength) {//Comparing indices based on words same length
i = i + wordLength – 1;
break;
} else {
i = indecies.get(n);
validSeq = true;
}
}
if (validSeq) {
returnedIndecies.add(indecies.get(0));
}

}
return returnedIndecies;

}
Larry Okeke

A simple to understand soluton.

Just get every possible concatenation of words in words array, and check whether each occurs in the main string.
if the result is not -1, we have an index of substring!

import java.util.*; public class duplicate_substring{ public static ArrayList allPossibleArrangements = new ArrayList(); public static void main(String[] args){ String[] arr = new String[] {"chinelo", "arukwe", "larry"}; ArrayList words = new ArrayList(Arrays.asList(arr)); String s = "chineloarukwelarryokekearukwechinelolarrytestinglarryarukwechinelo"; permutate(words, new Stack(), arr.length); List solution = solution(allPossibleArrangements.toArray(new String[0]), s); System.out.println(solution.toString()); } public static List solution(String[] words, String s){ //use combinatorial algorithm to get every possible concatenation of words //find the index of each derived word in s. StringBuilder build = new StringBuilder(); ArrayList result = new ArrayList(); for(int i = 0; i < words.length; i++){ String currentConcatenation = words[i]; if(s.indexOf(currentConcatenation)!=-1){ result.add(s.indexOf(currentConcatenation)); } build.setLength(0); } return result; } public static void permutate(List items, Stack permutation, int size) { /* permutation stack has become equal to size that we require */ if(permutation.size() == size) { /* print the permutation */ //System.out.println(Arrays.toString(permutation.toArray(new Integer[0]))); String s = ""; for(String str: permutation){ s+=str; } allPossibleArrangements.add(s); } /* items available for permutation */ String[] availableItems = items.toArray(new String[0]); for(String i : availableItems) { /* add current item */ permutation.push(i); /* remove item from available item set */ items.remove(i); /* pass it on for next permutation */ permutate(items, permutation, size); /* pop and put the removed item back */ items.add(permutation.pop()); } } }

LeetCode – Substring with Concatenation of All Words (Java)

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