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Problem solution · Go

Codeforces 1409F — Subsequences of Length Two

Codeforces 1409F — Subsequences of Length Two: a Go solution using dynamic programming. Learn the idea, check the complexity, and read the full code, with credit to EndlessCheng Codeforces Go.

Technique
Dynamic programming
Source
EndlessCheng Codeforces Go
Length
66 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Dynamic programming

For Codeforces 1409F — Subsequences of Length Two, the implementation records answers for smaller states and reuses them to build the requested result without repeating work.

  1. Define precisely what one DP state represents.
  2. Establish the base cases before transitions are evaluated.
  3. Process states in dependency order and combine only already-known values.

Code notes

  • 66 lines of Go from the credited upstream file 1409F.go.
  • The implementation visibly relies on sequence storage, cached states.
  • No explicit loop blocks detected.

Complexity

Multiply the number of reachable states by the work performed for each transition, then include the stored state table in memory usage.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from EndlessCheng Codeforces Go by Σndless (EndlessCheng) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeCodeforces 1409F — Subsequences of Length Two · GoGo
Use this to learn the idea, then write your own version.
package main import (	. "fmt"	"io"	"strings") // github.com/EndlessCheng/codeforces-gofunc CF1409F(in io.Reader, out io.Writer) {	var n, K int	var s, t string	Fscan(in, &n, &K, &s, &t)	if t[0] == t[1] {		c := strings.Count(s, t[:1]) + K		if c > n {			c = n		}		Fprint(out, c*(c-1)/2)		return	}	I := func(b bool) int {		if b {			return 1		}		return 0	}	max := func(a, b int) int {		if a > b {			return a		}		return b	} 	dp := make([][][]int, n)	for i := range dp {		dp[i] = make([][]int, K+1)		for j := range dp[i] {			dp[i][j] = make([]int, n+1)			for k := range dp[i][j] {				dp[i][j][k] = -1			}		}	}	var f func(p, left, c0 int) int	f = func(p, left, c0 int) (res int) {		if p == n {			return		}		dv := &dp[p][left][c0]		if *dv >= 0 {			return *dv		}		defer func() { *dv = res }()		res = I(s[p] == t[1])*c0 + f(p+1, left, c0+I(s[p] == t[0])) // 不修改		if left > 0 {			res = max(res, f(p+1, left-1, c0+1))  // 修改成 t[0]			res = max(res, c0+f(p+1, left-1, c0)) // 修改成 t[1]		}		return	}	Fprint(out, f(0, K, 0))} //func main() { CF1409F(os.Stdin, os.Stdout) }

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