# Algorithm - KMP

• What is prefix table?

• Table’s length is the length of pattern.
• Each value is the max length of substring that can fit as prefix and suffix.
• Prefix are substrings that don’t include the last char.
• Suffix are the substrings that don’t include the first char.
• For example aaba, next == 1 as in a.
• How to get the prefix table?

• i can be understood as the end of prefix.
• j can be understood as the end of suffix.
• Let i = 0, j = 1. Take aaa for example, if pattern[i] === pattern[j], then next[j] === i + 1.
• if pattern[i] !== pattern[j] like below
• • We need to find a max prefix for suffix (ABABA) ending at j! (The prefix will occupy all or part of ABABA)
• We need to go back, but apparently we cannot do i = next[i] because then we’ll always have a prefix ending with the same char like pattern[j].
• What about next[i - 1]? That sounds right!
• By the definition of this prefix table, pattern[0, i - 1] is the same as a string ending before j whose length is the same as pattern[0, i - 1]. Right now they both end with B.
• By next[i - 1], we will find (if it exsits) a max prefix that ends with B (even if we have to iterate multiple times).
• When we find this prefix, we get its length by next[]. And because of the table is indexed by 0. When pattern[j] === pattern[i], if means we are actually checking the char after the prefix.
• If they match, we’ve found a qualified prefix that’s the longest and it is followed by a char that is the same as pattern[j].
• We got the prefix!
• The techqinue:

• Deal with situations where chars are not equal first.
• Use while loops.
• Try to remember it. A algo from 1977 is now a leetcode easy question, what a world.