go - 为什么 slice 值会改变？

Question

附加到二维 slice 的 slice 的值稍后会更改。为什么会发生？

完整代码如下:

import "sort"
import "fmt"

func combinationSum(candidates []int, target int) [][]int {
    sort.Ints(candidates)
    var ret [][]int
    var curlist []int
    combinationSumRecursive(candidates, target, 0, curlist, &ret)
    fmt.Println("ret", ret)
    return ret
}

func combinationSumRecursive(candidates []int, target int, curCandidateIdx int, curlist []int, ret *[][]int) {
    if target == 0 {
        fmt.Println("put", curlist)
        *ret = append(*ret, curlist)
        return
    }
    for i:=curCandidateIdx;i<len(candidates);i++{
        if candidates[i] > target {
            break
        }
        combinationSumRecursive(candidates, target - candidates[i], i, append(curlist, candidates[i]), ret)
    }
}

输入是:

[7,3,2]
18

输出如下:

put [2 2 2 2 2 2 2 2 2]
put [2 2 2 2 2 2 3 3]
put [2 2 2 2 3 7]
put [2 2 2 3 3 3 3]
put [2 2 7 7]
put [2 3 3 3 7]
put [3 3 3 3 3 3]
ret [[2 2 2 2 2 2 2 2 2] [2 2 2 2 2 7 3 3] [2 2 2 2 3 7] [2 2 2 3 3 3 3] [2 2 7 7] [2 3 3 3 7] [3 3 3 3 3 3]]

ret 中的第二个 slice 已更改。 [2 2 2 2 2 2 3 3] 是预期的。

感谢任何帮助。

Answer 1

这是由于 combinationSumRecursive(candidates, target - candidates[i], i, append(curlist, candidates[i]) 行的 append 中 slice curlist 的变化引起的, ret).

一个解决方案是在将附加结果传递到下一个递归级别之前将其重新分配给 curlist。

package main

import "sort"
import "fmt"

func combinationSum(candidates []int, target int) [][]int {
    sort.Ints(candidates)
    var ret [][]int
    var curlist []int
    combinationSumRecursive(candidates, target, 0, curlist, &ret)
    fmt.Println("ret", ret)
    return ret
}

func combinationSumRecursive(candidates []int, target int, curCandidateIdx int, curlist []int, ret *[][]int) {
    if target == 0 {
        fmt.Println("put", curlist)
        *ret = append(*ret, curlist)
        return
    }
    for i := curCandidateIdx; i < len(candidates); i++ {
        if candidates[i] > target {
            break
        }
        curlist = append(curlist, candidates[i])
        combinationSumRecursive(candidates, target-candidates[i], i, curlist, ret)
    }
}

func main() {
    combinationSum([]int{7, 3, 2}, 18)
}

这会产生:

put [2 2 2 2 2 2 2 2 2]
put [2 2 2 2 2 2 3 3]
put [2 2 2 2 3 7]
put [2 2 2 3 3 3 3]
put [2 2 7 7]
put [2 3 3 3 7]
put [3 3 3 3 3 3]
ret [[2 2 2 2 2 2 2 2 2] [2 2 2 2 2 2 3 3] [2 2 2 2 3 7] [2 2 2 3 3 3 3] [2 2 7 7] [2 3 3 3 7] [3 3 3 3 3 3]]

[编辑] 首先阅读 this .这解释了虽然您看到 slice header 的地址发生了变化，但在追加之后后备数组仍然可以保持不变。

然后 this是您的案例中发生的情况的示例。由于递归调用和将当前索引重新定位到 curlist 中，您的问题很难看出来。