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Program to find number of different substrings of a string for different queries in Python
Suppose we have a string s whose length is n. We also have a list of queries Q, where Q[i] contains a pair (l, r). For each query we have to count number of different substrings of s in the inclusive range between l and r.
So, if the input is like s = "ppqpp" Q = [(1,1),(1,4),(1,1),(0,2)], then the output will be [1,8,1,5] because
For query (1, 1) the only substring is 'p' so output is 1
For query (1, 4) the substrings are 'p', 'q', 'pq', 'qp', 'pp', 'pqp', 'qpp' and 'pqpp', so output is 8
Again for query (1, 1) the only substring is 'p' so output is 1
For query (0, 2) the substrings are 'p', 'q', 'pp', 'pq', 'ppq', so the output is 8.
To solve this, we will follow these steps −
- Define a function kasai() . This will take s, suff, n
- lcp := an array of size n and fill with 0
- inv := an array of size n and fill with 0
- for i in range 0 to n - 1, do
- inv[suff [i]] := i
- k := 0
- for i in range 0 to n - 1, do
- if inv [i] is same as n-1, then
- k := 0
- go for next iteration
- j := suff[inv [i] + 1]
- while i + k < n and j + k < n and s[i + k] is same as s[j + k], do
- k := k + 1
- lcp[inv [i]] := k
- if k > 0, then
- k := k - 1
- if inv [i] is same as n-1, then
- return lcp
- From the main method, do the following −
- res := a new list
- for i in range 0 to size of Q - 1, do
- (left, right) := Q[i]
- sub := substring of s from index left to right
- length := right-left + 1
- suffix := a list of pairs (i, substring of sub from index i to end) for each i in range 0 to length - 1
- sort suffix based on the second item of the pair substring
- (suff, suffix) = the pairs of indices and corresponding substrings from suffix
- lcp := kasai(sub, suff, length)
- count := size of suffix[0]
- for i in range 0 to length-2, do
- count := count + size of suffix[i + 1] - lcp[i]
- insert count at the end of res
- return res
Example
Let us see the following implementation to get better understanding −
def kasai (s, suff, n): lcp = [0] * n inv = [0] * n for i in range (n): inv [suff [i]] = i k = 0 for i in range (n): if inv [i] == n-1: k = 0 continue j = suff [inv [i] + 1] while i + k <n and j + k <n and s [i + k] == s [j + k]: k += 1 lcp [inv [i]] = k if k> 0: k -= 1 return lcp def solve(s, Q): res = [] for i in range (len(Q)): left, right = Q[i] sub = s [left: right + 1] length = right-left + 1 suffix = [[i, sub [i:]] for i in range (length)] suffix.sort (key = lambda x: x [1]) suff, suffix = [list (t) for t in zip (* suffix)] lcp = kasai (sub, suff, length) count = len (suffix [0]) for i in range (length-1): count += len (suffix [i + 1]) - lcp [i] res.append(count) return res s = "pptpp" Q = [(1,1),(1,4),(1,1),(0,2)] print(solve(s, Q))
Input
"pptpp", [(1,1),(1,4),(1,1),(0,2)]
Output
[1, 8, 1, 5]
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