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Problem solution · Go

Codeforces 1771D — Hossam and (sub-)palindromic tree

Codeforces 1771D — Hossam and (sub-)palindromic tree: 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
93 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 1771D — Hossam and (sub-)palindromic tree, 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

  • 93 lines of Go from the credited upstream file 1771D.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 1771D — Hossam and (sub-)palindromic tree · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "fmt"	"io") // https://space.bilibili.com/206214func CF1771D(_r io.Reader, _w io.Writer) {	in := bufio.NewReader(_r)	out := bufio.NewWriter(_w)	defer out.Flush()	max := func(a, b int) int {		if b > a {			return b		}		return a	} 	var T, n, v, w int	var s []byte	for Fscan(in, &T); T > 0; T-- {		Fscan(in, &n, &s)		g := make([][]int, n)		for i := 1; i < n; i++ {			Fscan(in, &v, &w)			v--			w--			g[v] = append(g[v], w)			g[w] = append(g[w], v)		} 		move1 := make([][]int, n)		for i := range move1 {			move1[i] = make([]int, n)		}		for rt := range move1 {			var build func(int, int)			build = func(v, fa int) {				move1[v][rt] = fa				for _, w := range g[v] {					if w != fa {						build(w, v)					}				}			}			build(rt, rt)		} 		dp := make([][]int, n)		for i := range dp {			dp[i] = make([]int, n)			for j := range dp[i] {				dp[i][j] = -1			}		}		var f func(int, int) int		f = func(v, w int) (res int) {			if v > w { // 神优化				v, w = w, v			}			if v == w {				return 1			}			if move1[v][w] == w {				if s[v] == s[w] {					return 2				}				return 1			}			dv := &dp[v][w]			if *dv != -1 {				return *dv			}			defer func() { *dv = res }()			if s[v] == s[w] {				return 2 + f(move1[v][w], move1[w][v])			}			return max(f(move1[v][w], w), f(v, move1[w][v]))		}		ans := 0		for i := 0; i < n; i++ {			for j := i; j < n; j++ {				ans = max(ans, f(i, j))			}		}		Fprintln(out, ans)	}} //func main() { CF1771D(os.Stdin, os.Stdout) }

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