# Substring with highest frequency length product

Given a string which contains lower alphabetic characters, we need to find out such a substring of this string whose product of length and frequency in string is maximum among all possible choices of substrings.
Examples:

Input : String str = “abddab”
Output : 6
All unique substring with product of their
frequency and length are,
Val["a"] = 2 * 1 = 2
Val["ab"] = 2 * 2 = 4
Val["abd"] = 1 * 3 = 3
Val["abdd"] = 1 * 4 = 4
Val["abdda"] = 1 * 5 = 5
Val["abddab"] = 1 * 6 = 6
Val["b"] = 2 * 1 = 2
Val["bd"] = 1 * 2 = 2
Val["bdd"] = 1 * 3 = 3
Val["bdda"] = 1 * 4 = 4
Val["bddab"] = 1 * 5 = 5
Val["d"] = 2 * 1 = 2
Val["da"] = 1 * 2 = 2
Val["dab"] = 1 * 3 = 3
Val["dd"] = 1 * 2 = 2
Val["dda"] = 1 * 3 = 3
Val["ddab"] = 1 * 4 = 4

Input  : String str = “zzzzzz”
Output : 12
In above string maximum value 12 can
be obtained with substring “zzzz”

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

A simple solution is to consider all substrings one by one. For every substring, count number of occurrences of it in whole string.

An efficient solution to solve this problem by first constructing longest common prefix array, now suppose value of lcp[i] is K then we can say that i-th and (i+1)-th suffix has K length prefix in common i.e. there is a substring of length K which is repeating twice. In the same way, let three consecutive values of lcp are (K, K-2, K+1) then we can say that there is a substring of length (K-2) which is repeating three times in the string.
Now after above observation, we can see that our result will be such a range of lcp array whose smallest element times number of elements in the range is maximum because range will correspond to the frequency of string and smallest element of range will correspond to length of repeating string now this reformed problem can be solved similar to largest rectangle in histogram problem.
In below code lcp array is constructed by Kasai’s algorithm.

 // C++ program to find substring with highest // frequency length product #include using namespace std;    // Structure to store information of a suffix struct suffix {     int index;  // To store original index     int rank[2]; // To store ranks and next rank pair };    // A comparison function used by sort() to compare // two suffixes. Compares two pairs, returns 1 if // first pair is smaller int cmp(struct suffix a, struct suffix b) {     return (a.rank[0] == b.rank[0])?            (a.rank[1] < b.rank[1] ?1: 0):            (a.rank[0] < b.rank[0] ?1: 0); }    // This is the main function that takes a string // 'txt' of size n as an argument, builds and // return the suffix array for the given string vector buildSuffixArray(string txt, int n) {     // A structure to store suffixes and their indexes     struct suffix suffixes[n];        // Store suffixes and their indexes in an array     // of structures. The structure is needed to sort     // the suffixes alphabatically and maintain their     // old indexes while sorting     for (int i = 0; i < n; i++)     {         suffixes[i].index = i;         suffixes[i].rank[0] = txt[i] - 'a';         suffixes[i].rank[1] = ((i+1) < n)? (txt[i + 1] - 'a'): -1;     }        // Sort the suffixes using the comparison function     // defined above.     sort(suffixes, suffixes+n, cmp);        // At his point, all suffixes are sorted according to first     // 2 characters.  Let us sort suffixes according to first 4     // characters, then first 8 and so on     // This array is needed to get the index in suffixes[]     // from original index.  This mapping is needed to get     // next suffix.     int ind[n];     for (int k = 4; k < 2*n; k = k*2)     {         // Assigning rank and index values to first suffix         int rank = 0;         int prev_rank = suffixes[0].rank[0];         suffixes[0].rank[0] = rank;         ind[suffixes[0].index] = 0;            // Assigning rank to suffixes         for (int i = 1; i < n; i++)         {             // If first rank and next ranks are same as             // that of previous suffix in array, assign             // the same new rank to this suffix             if (suffixes[i].rank[0] == prev_rank &&                     suffixes[i].rank[1] == suffixes[i-1].rank[1])             {                 prev_rank = suffixes[i].rank[0];                 suffixes[i].rank[0] = rank;             }             else // Otherwise increment rank and assign             {                 prev_rank = suffixes[i].rank[0];                 suffixes[i].rank[0] = ++rank;             }             ind[suffixes[i].index] = i;         }            // Assign next rank to every suffix         for (int i = 0; i < n; i++)         {             int nextindex = suffixes[i].index + k/2;             suffixes[i].rank[1] = (nextindex < n)?                  suffixes[ind[nextindex]].rank[0]: -1;         }            // Sort the suffixes according to first k characters         sort(suffixes, suffixes+n, cmp);     }        // Store indexes of all sorted suffixes in the suffix array     vectorsuffixArr;     for (int i = 0; i < n; i++)         suffixArr.push_back(suffixes[i].index);        // Return the suffix array     return  suffixArr; }    /* To construct and return LCP */ vector kasai(string txt, vector suffixArr) {     int n = suffixArr.size();        // To store LCP array     vector lcp(n, 0);        // An auxiliary array to store inverse of suffix array     // elements. For example if suffixArr[0] is 5, the     // invSuff[5] would store 0.  This is used to get next     // suffix string from suffix array.     vector invSuff(n, 0);        // Fill values in invSuff[]     for (int i=0; i < n; i++)         invSuff[suffixArr[i]] = i;        // Initialize length of previous LCP     int k = 0;        // Process all suffixes one by one starting from     // first suffix in txt[]     for (int i=0; i0)             k--;     }        // return the constructed lcp array     return lcp; }    //    method to get LCP array vector getLCPArray(string str) {     vectorsuffixArr = buildSuffixArray(str, str.length());     return kasai(str, suffixArr); }    // The main function to find the maximum rectangular // area under given histogram with n bars int getMaxArea(int hist[], int n) {     // Create an empty stack. The stack holds indexes     // of hist[] array. The bars stored in stack are     // always in increasing order of their heights.     stack s;        int max_area = 0; // Initialize max area     int tp;  // To store top of stack        // To store area with top bar as the smallest bar     int area_with_top;        // Run through all bars of given histogram     int i = 0;     while (i < n)     {         // If this bar is higher than the bar on         // top stack, push it to stack         if (s.empty() || hist[s.top()] <= hist[i])             s.push(i++);            // If this bar is lower than top of stack,         // then calculate area of rectangle with         // stack top as the smallest (or minimum         // height) bar. 'i' is 'right index' for         // the top and element before top in stack         // is 'left index'         else         {             tp = s.top();  // store the top index             s.pop();  // pop the top                // Calculate the area with hist[tp]             // stack as smallest bar             area_with_top = hist[tp] * (s.empty() ?                            (i + 1) : i - s.top());                // update max area, if needed             if (max_area < area_with_top)                 max_area = area_with_top;         }     }        // Now pop the remaining bars from stack     // and calculate area with every     // popped bar as the smallest bar     while (s.empty() == false)     {         tp = s.top();         s.pop();         area_with_top = hist[tp] * (s.empty() ?                         (i + 1) : i - s.top());            if (max_area < area_with_top)             max_area = area_with_top;     }        return max_area; }    // Returns maximum product of frequency and length // of a substring. int maxProductOfFreqLength(string str) {     //    get LCP array by Kasai's algorithm     vector lcp = getLCPArray(str);        int hist[lcp.size()];        //    copy lcp array into hist array     for (int i = 0; i < lcp.size(); i++)         hist[i] = lcp[i];        //    get the maximum area under lcp histogram     int substrMaxValue = getMaxArea(hist, lcp.size());        // if string length itself is greater than     // histogram area, then return that     if (str.length() > substrMaxValue)         return str.length();     else         return substrMaxValue; }    // Driver code to test above methods int main() {     string str = "abddab";     cout << maxProductOfFreqLength(str) << endl;     return 0; }

Output:

6

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