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Problem solution · Go

Codeforces 706C — Hard problem

Codeforces 706C — Hard problem: a Go solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to EndlessCheng Codeforces Go.

Technique
Direct simulation
Source
EndlessCheng Codeforces Go
Length
108 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

Direct simulation

For Codeforces 706C — Hard problem, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 108 lines of Go from the credited upstream file 706C.go.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

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 706C — Hard problem · GoGo
Use this to learn the idea, then write your own version.
package main import (	"bufio"	. "container/heap"	. "fmt"	"io") type hPair706C struct {	x int64	y int}type pairHeap706C []hPair706C func (h pairHeap706C) Len() int              { return len(h) }func (h pairHeap706C) Less(i, j int) bool {	return h[i].x < h[j].x || h[i].x == h[j].x && h[i].y < h[j].y}func (h pairHeap706C) Swap(i, j int)         { h[i], h[j] = h[j], h[i] }func (h *pairHeap706C) Push(v interface{})   { *h = append(*h, v.(hPair706C)) }func (h *pairHeap706C) Pop() (v interface{}) { n := len(*h); *h, v = (*h)[:n-1], (*h)[n-1]; return } // github.com/EndlessCheng/codeforces-gofunc Sol706C(reader io.Reader, writer io.Writer) {	const inf int64 = 1e18	reverse := func(ss string) string {		n := len(ss)		s := make([]byte, n)		for i := range s {			s[i] = ss[n-1-i]		}		return string(s)	}	in := bufio.NewReader(reader)	out := bufio.NewWriter(writer)	defer out.Flush() 	var n int	Fscan(in, &n)	costs := make([]int, n)	for i := range costs {		Fscan(in, &costs[i])	}	type neighbor struct {		vertex int		weight int	}	g := make([][]neighbor, 2*n+2)	var prev, rPrev, cur, rCur string	for i, c := range costs {		Fscan(in, &cur)		rCur = reverse(cur)		if prev == "" {			g[0] = []neighbor{{1, 0}, {2, c}}		} else {			if prev <= cur {				g[2*i-1] = append(g[2*i-1], neighbor{2*i + 1, 0})			}			if prev <= rCur {				g[2*i-1] = append(g[2*i-1], neighbor{2*i + 2, c})			}			if rPrev <= cur {				g[2*i] = append(g[2*i], neighbor{2*i + 1, 0})			}			if rPrev <= rCur {				g[2*i] = append(g[2*i], neighbor{2*i + 2, c})			}		}		prev, rPrev = cur, rCur	}	g[2*n-1] = []neighbor{{2*n + 1, 0}}	g[2*n] = []neighbor{{2*n + 1, 0}} 	dist := make([]int64, 2*n+2)	for i := range dist {		dist[i] = inf	}	dist[0] = 0	visited := make([]bool, 2*n+2)	h := &pairHeap706C{}	Push(h, hPair706C{0, 0})	for h.Len() > 0 {		p := Pop(h).(hPair706C)		v := p.y		if visited[v] {			continue		}		visited[v] = true		for _, e := range g[v] {			w := e.vertex			if newDist := dist[v] + int64(e.weight); newDist < dist[w] {				dist[w] = newDist				Push(h, hPair706C{newDist, w})			}		}	}	ans := dist[2*n+1]	if ans == inf {		ans = -1	}	Fprintln(out, ans)} //func main() {//	Sol706C(os.Stdin, os.Stdout)//}

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