Rare
0/39

# String Searching

Authors: Benjamin Qi, Siyong Huang, Dustin Miao, Mihnea Brebenel

Knuth-Morris-Pratt and Z Algorithms (and a few more related topics).

### Prerequisites

Resources
CPC

String Matching, KMP, Tries

CP2

# Single String

### A Note on Notation:

For a string $S$:

• $|S|$ denotes the size of string $S$
• $S[i]$ denotes the character at index $i$ starting from $0$
• $S[l:r]$ denotes the substring beginning at index $l$ and ending at index $r$
• $S[:r]$ is equivalent to $S[0:r]$, represents the prefix ending at $r$
• $S[l:]$ is equivalent to $S[l:|S| - 1]$, represents the suffix beginning of $l$.
• $S + T$ denotes concactinating $T$ to the end of $S$. Note that this implies that addition is non-commutative.

## Knuth-Morris-Pratt Algorithm

Define an array $\pi_S$ of size $|S|$ such that $\pi_S[i]$ is equal to the length of the longest nontrivial suffix of the prefix ending at position $i$ that coincides with a prefix of the entire string. Formally,

$\pi_S[i] = \max \{k \: | \: 1 \leq k < i \text{ and } S[0:k - 1] \equiv S[i - (k - 1): i] \}$

In other words, for a given index $i$, we would like to compute the length of the longest substring that ends at $i$, such that this string also happens to be a prefix of the entire string. One such string that satisfies this criteria is the prefix ending at $i$; we will be disregarding this solution for obvious reasons.

For instance, for $S = \text{abcabcd"}$, $\pi_S = [0, 0, 0, 1, 2, 3, 0]$, and the prefix function of $S = \text{aabaaab"}$ is $\pi_S = [0, 1, 0, 1, 2, 2, 3]$. In the second example, $\pi_S[4] = 2$ because the prefix of length $2$ ($\text{ab"})$ is equivalent to the substring of length $2$ that ends at index $4$. In the same way, $\pi_S[6] = 3$ because the prefix of length $3$ ($\text{abb"}$) is equal to the substring of length $3$ that ends at index $6$. For both of these samples, there is no longer substring that satisfies these criteria.

The purpose of the KMP algorithm is to efficiently compute the $\pi_S$ array in linear time. Suppose we have already computed the $\pi_S$ array for indices $0\dots i$, and need to compute the value for index $i + 1$.

Firstly, note that between $\pi_S[i]$ and $\pi_S[i + 1]$, $\pi_S[i + 1]$ can be at most one greater. This occurs when $S[\pi_S[i]] = S[i + 1]$.

In the example above, $\pi_S[i] = 5$, meaning that the suffix of length $5$ is equivalent to a prefix of length $5$ of the entire string. It follows that if the character at position $5$ of the string is equal to the character at position $i + 1$, then the match is simply extended by a single character. Thus, $\pi_S[i + 1] = \pi_S[i] + 1 = 6$.

In the general case, however, this is not necessarily true. That is to say, $S[\pi_S[i]] \neq S[i + 1]$. Thus, we need to find the largest index $j < \pi_S[i]$ such that the prefix property holds (ie $S[:j - 1] \equiv S[i - j + 1:i]$). For such a length $j$, we repeat the procedure in the first example by comparing characters at indicies $j$ and $i + 1$: if the two are equal, then we can conclude our search and assign $\pi_S[i + 1] = j + 1$, and otherwise, we find the next smallest $j$ and repeat. Indeed, notice that the first example is simply the case where $j$ begins as $\pi_S[i]$.

In the second example above, we let $j = 2$.

The only thing that remains is to be able to efficiently find all the $j$ that we might possibly need. To recap, if the position we're currently at is $j$, to handle transitions we need to find the largest index $k$ that satisfies the prefix property $S[:k - 1] \equiv S[j - k + 1 : j]$. Since $j < i$, this value is simply $\pi_S[j - 1]$, a value that has already been computed. All that remains is to handle the case where $j = 0$. If $S[0] = S[i + 1]$, $\pi_S[i + 1] = 1$, otherwise $\pi_S[i + 1] = 0$.

C++

vector<int> pi(const string &s) {	int n = (int)s.size();	vector<int> pi_s(n);	for (int i = 1, j = 0; i < n; i++) {		while (j > 0 && s[j] != s[i]) { j = pi_s[j - 1]; }		if (s[i] == s[j]) { j++; }		pi_s[i] = j;	}	return pi_s;}

Python

from typing import List

def pi(s: str) -> List[int]:	n = len(s)	pi_s = [0] * n	j = 0	for i in range(1, n):		while j > 0 and s[j] != s[i]:			j = pi_s[j - 1]		if s[i] == s[j]:			j += 1		pi_s[i] = j	return pi_s

Claim: The KMP algorithm runs in $\mathcal{O}(n)$ for computing the $\pi_S$ array on a string $S$ of length $n$.

Proof: Note that $j$ doesn't actually change through multiple iterations. This is because on iteration $i$, we assign $j = \pi_S[i - 1]$. However, in the previous iteration, we assign $\pi_S[i - 1]$ to be $j$. Furthermore, note that $j$ is always non-negative. In each iteration of $i$, $j$ is only increased by at most $1$ in the if statement. Since $j$ remains non-negative and is only increased a constant amount per iteration, it follows that $j$ can only decrease by at most $n$ times through all iterations of $i$. Since the inner loop is completely governed by $j$, the overall complexity amortizes to $\mathcal{O}(n)$. $\blacksquare$

### Problems

StatusSourceProblem NameDifficultyTags
CSESVery Easy
Show TagsKMP, Z
POIEasy
Show TagsKMP, Strings
Baltic OINormal
Show TagsKMP, Strings
Old GoldHard
Show TagsKMP, Strings
POIHard
Show TagsKMP, Strings
CEOIHard
Show TagsKMP
POIVery Hard
Show TagsKMP
POIVery Hard
Show TagsKMP

## Z Algorithm

Focus Problem – try your best to solve this problem before continuing!

### Explanation

The Z-algorithm is very similar to KMP, but it uses a different function than $\pi$ and it has an interesting different application than string matching.

Instead of using $\pi$, it uses the z-function. Given a position, this function gives the length of the longest string that's both the prefix of $S$ and of the suffix of $S$ starting at the given position.

Here's some examples of what this function might look like:

• aabxaayaab $\rightarrow$ $Z=[10,1,0,0,2,1,0,3,1,0]$
• aabxaabxcaabxaabxay $\rightarrow$ $Z=[18,1,0,0,4,1,0,0,0,8,1,0,0,5,1,0,0,1,0]$

Let's also take a closer look at $Z_9=8$ (0-indexed) for the second string. The value for this position if $8$ because that's the longest common prefix between the string itself aabxaabxcaabxaabxay and the suffix starting at position $9$ aabxaabxay (also 0-indexed).

To efficiently compute this array, we maintain the $[l, r]$ interval such that $S_{l...r}$ is also a prefix, i.e. $Z_l=r-l+1$.

Say we have a position $i$ anywhere in $[l,r]$. We would then have these two cases:

1. If $i + Z_{i-l} < r$, we know that $Z_i = Z_{i-l}$.
2. Otherwise, $i + Z_{i-l} \geq r$, meaning that the answer can expand beyond $r$. Thus, we compare character by character from there on.

### Implementation

Time Complexity: $\mathcal{O}(N)$

C++

#include <bits/stdc++.h>using namespace std;
int main() {	string s;	cin >> s;	vector<int> z(s.size());
for (int i = 1, l = 0, r = 0; i < s.size(); i++) {		z[i] = max(0, min(z[i - l], r - i + 1));
StatusSourceProblem NameDifficultyTags
YSVery Easy
Show TagsZ
CSESVery Easy
Show TagsKMP, Z
CFNormal
Show TagsDP, Strings
CFNormal
Show TagsZ
CFHard

# Palindromes

## Manacher

Focus Problem – try your best to solve this problem before continuing!

Manacher's Algorithm functions similarly to the Z-Algorithm. It determines the longest palindrome centered at each character.

Let's denote $dp_i$ as the maximum diameter of a palindrome centered at $i$. Manacher's algorithm makes use of the previously determined $dp_j$, where $j < i$ incalculating $dp_i$. The main idea is that for a palindrome centered at $i$ with the borders $left$ and $right$ the $dp_j$ ($i < j \le right$) values are - probably - mirrors of the $dp_k$ ($left \le k < i$) values on the left side of the palindrome. Probably because for some $j$ the maximum palindrome might cross the right border. This way the algorithm only considers the palindrome centers that could lead to the expansion of the maximum palindrome.

Time complexity: $\mathcal{O}(N)$

C++

#include <bits/stdc++.h>using namespace std;
string menacher(string s) {	// Preprocess the input so it can handle even length palindromes	string arr;	for (int i = 0; i < s.size(); i++) {		arr.push_back('#');		arr.push_back(s[i]);	}

### Don't Forget!

If s[l, r] is a palindrome, then s[l+1, r-1] is as well.

StatusSourceProblem NameDifficultyTags
CFNormal
Show TagsStrings
CFNormal
Show TagsStrings
CFHard
Show TagsPrefix Sums, Strings

## Palindromic Tree

A Palindromic Tree is a tree-like data structure that behaves similarly to KMP. Unlike KMP, in which the only empty state is $0$, the Palindromic Tree has two empty states: length $0$, and length $-1$. This is because appending a character to a palindrome increases the length by $2$, meaning a single character palindrome must have been created from a palindrome of length $-1$

StatusSourceProblem NameDifficultyTags
APIOEasy
CFHard
Show TagsPrefix Sums, Strings
MMCCVery Hard

# Multiple Strings

## Tries

Focus Problem – try your best to solve this problem before continuing!

A trie is a tree-like data structure that stores strings. Each node is a string, and each edge is a character.

The root is the empty string, and every node is represented by the characters along the path from the root to that node. This means that every prefix of a string is an ancestor of that string's node.

C++

#include <bits/stdc++.h>using namespace std;
const int NMAX = 5e3;const int WMAX = 1e6;const int MOD = 1e9 + 7;
int trie[WMAX][26];int node_count;bool stop[WMAX];
StatusSourceProblem NameDifficultyTags
COCIVery Easy
Show TagsDFS, Strings, Trie
IOIVery Easy
Show TagsDFS, Strings, Trie
CFNormal
Show TagsMST, Trie
YSEasy
Show TagsGreedy, Trie
GoldNormal
Show TagsStrings, Trie
CFNormal
Show TagsStrings, Trie
COCINormal
Show TagsTrie
ACNormal
CFNormal
Show TagsSmall to Large, Tree, Trie
CFNormal
IZhOHard
Show TagsGreedy, Trie
JOIHard
Show TagsBIT, Trie
CFHard
Show TagsTree, Trie

## Aho-Corasick

Focus Problem – try your best to solve this problem before continuing!

The Aho-Corasick algorithm stores the pattern words in a trie structure, described above. It uses the trie to transition from a state to anoother. Similar to KMP algorithm, we want to reuse the information we have already processed.

A suffix link or failure link for a node $u$ is a special edge that points to the longest proper suffix of the string corresponding to node $u$. The suffix links for the root and all its immediate children point to the root node. For all other nodes $u$ with parent $p$ and letter $c$ on the edge $p \rightarrow u$ the suffix link can be computed by following $p$'s failure link and transitioning to letter $c$ from there.

While processing the string $S$ the algorithm maintains the current node in the trie such that word formed in the node is equal to the longest suffix ending in $i$.

For example, when transitioning from $i$ to $i+1$ in $S$ there only are two choices:

1. If $node$ does have an outgoing edge with letter $S_{i+1}$, then move down the edge.
2. Otherwise, follow the failure link of $S_i$ and transition to letter $S_{i+1}$ from there.

The image below depicts how the structure looks like for the words $[a, ag, c, caa, gag, gc]$.

An Aho-Corasick trie with failure links as light edges.

There is a special case when some words are substring of other words in the wordlist. This could lead to some problems depending on the implementation. Dictionary links can solve this problem. They act like suffix links that point to the first suffix that is also a word in the wordlist. The code below constructs the structure using a BFS.

Time Complexity: $\mathcal{O}(m\sigma)$ - where $m$ is the size of the alphaber and $\sigma$ the size of the alphabet

C++

#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 6e5;const int SIGMA = 26;
int n;string s;// The number of nodes in trie
StatusSourceProblem NameDifficultyTags
CFEasy
Show TagsStrings
GoldNormal
Show TagsStrings
CFNormal
Show TagsStrings

### This section is not complete.

Any help would be appreciated! Just submit a Pull Request on GitHub.

1731 Word Combinations -> trie

1732 Finding Borders -> string search

1733 Finding Periods -> string search

1110 Minimal Rotation -> string search

1111 Longest Palindrome -> string search

1112 Required Substring -> string search

### Join the USACO Forum!

Stuck on a problem, or don't understand a module? Join the USACO Forum and get help from other competitive programmers!