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Problem solution · Go

Codeforces 453B — Little Pony and Harmony Chest

Codeforces 453B — Little Pony and Harmony Chest: 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
70 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 453B — Little Pony and Harmony Chest, 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

  • 70 lines of Go from the credited upstream file 453B.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 453B — Little Pony and Harmony Chest · GoGo
Use this to learn the idea, then write your own version.
package main import (	. "fmt"	"io") // github.com/EndlessCheng/codeforces-gofunc CF453B(in io.Reader, out io.Writer) {	primes := [...]int{2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53}	const mx = len(primes)	const mx2 = 1 << mx	masks := [59]int{}	for i := 2; i < 59; i++ {		for j, p := range primes {			if i%p == 0 {				masks[i] |= 1 << j			}		}	}	abs := func(x int) int {		if x < 0 {			return -x		}		return x	} 	var n, v int	Fscan(in, &n)	dp := make([][mx2]int, n+1)	for i := range dp {		for j := 0; j < mx2; j++ {			dp[i][j] = 1e9		}	}	dp[0][0] = 0	type pair struct{ j, v int }	fa := make([][mx2]pair, n+1)	for i := 1; i <= n; i++ {		Fscan(in, &v)		for j := 1; j == 1 || j < v*2-1; j++ {			m := masks[j]			c := m ^ (mx2 - 1)			for sub, ok := c, true; ok; ok = sub != c {				if s := dp[i-1][sub] + abs(j-v); s < dp[i][m|sub] {					dp[i][m|sub] = s					fa[i][m|sub] = pair{sub, j}				}				sub = (sub - 1) & c			}		}	}	miJ := 0	for j, v := range dp[n] {		if v < dp[n][miJ] {			miJ = j		}	}	ans := make([]int, n)	for i, j := n, miJ; i > 0; i-- {		ans[i-1] = fa[i][j].v		j = fa[i][j].j	}	for _, v := range ans {		Fprint(out, v, " ")	}} //func main() { CF453B(os.Stdin, os.Stdout) }

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