Last active
September 27, 2024 21:38
Longest Common Subsequence using Tabulation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
func lcs(_ t1: [Character], _ t2: [Character]) -> Int { | |
// 1. build and default grid to 0 | |
let row = Array(repeating: 0, count: t2.count) | |
var grid = Array(repeating: row, count: t1.count) | |
func lcs(_ s: Stage) -> Int { | |
// 3. Exit with a 0 for any point that's beyond the left or bottom edges | |
guard s.s1 >= 0 && s.s2 >= 0 else { | |
return 0 | |
} | |
if t1[s.s1] == t2[s.s2] { | |
return (grid.safe(at:s.s1 - 1)?.safe(at:s.s2 - 1) ?? 0) + 1 | |
} else { | |
return max(grid.safe(at:s.s1 - 1)?[s.s2] ?? 0, grid[s.s1].safe(at:s.s2 - 1) ?? 0) | |
} | |
} | |
// 2. starting from top left corner: find the LCS value for every item in the grid. | |
// End at bottom right corner | |
for row in 0..<grid.count { | |
for column in 0..<grid[0].count { | |
grid[row][column] = lcs(Stage(row,column)) | |
} | |
} | |
// return the last result at the end of your tabulation | |
return grid[t1.count - 1][t2.count - 1] | |
} | |
extension Array { | |
func safe(at index: Int) -> Element? { | |
guard index < count && index >= 0 else { | |
return nil | |
} | |
return self[index] | |
} | |
} | |
struct Stage: Hashable { | |
var s1: Int | |
var s2: Int | |
init(_ s1: Int, _ s2: Int) { | |
self.s1 = s1 | |
self.s2 = s2 | |
} | |
} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment