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c.go
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c.go
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package main
// github.com/EndlessCheng/codeforces-go
type CombinationIterator struct{}
var (
s string
k int
idList [][]int
)
func combinations(n, r int, do func(ids []int)) {
ids := make([]int, r)
for i := range ids {
ids[i] = i
}
do(ids)
for {
i := r - 1
for ; i >= 0; i-- {
if ids[i] != i+n-r {
break
}
}
if i == -1 {
return
}
ids[i]++
for j := i + 1; j < r; j++ {
ids[j] = ids[j-1] + 1
}
do(ids)
// 可以加一个提前退出的逻辑(虽然最多 6435 个)
if len(idList) > 1e4 {
return
}
}
}
func Constructor(S string, K int) (_ CombinationIterator) {
s, k = S, K
idList = nil
combinations(len(s), k, func(ids []int) { idList = append(idList, append([]int(nil), ids...)) })
return
}
func (CombinationIterator) Next() string {
ids := idList[0]
idList = idList[1:]
ans := make([]byte, k)
for i, id := range ids {
ans[i] = s[id]
}
return string(ans)
}
func (CombinationIterator) HasNext() bool {
return len(idList) > 0
}