代码随想录算法训练营|day24

77.组合

以n=5,k=3为例

(1)for循环遍历，递归选择符合要求的值加入path，len(path)==k时，返回
statrtIndex保证每次递归取到的值不重复
剪枝：i<=n-(k-len(path))+1 后续需要k-len(path)个值，因为i可选区间左闭右闭，故+1

func combine(n int, k int) [][]int {
    res := [][]int{}
    path := []int{}
    var helper func(n, k, startIndex int)
    helper = func(n, k, startIndex int) {
        if len(path) == k {
            tmp := make([]int, k)
            copy(tmp, path)
            res = append(res, tmp)
            return
        }
        for i:= startIndex; i <= n; i++ {
            path = append(path, i)
            helper(n, k, i+1)
            path = path[:len(path) - 1]
        }
    }
    helper(n, k, 1)
    return res
}

(2)递归枚举每个值，选/不选
若不选当前值，递归下一个值；若选当前值，加入结果集，回溯，保证当前状态不影响下一次递归

func combine(n int, k int) [][]int {
    res := [][]int{}
    path := []int{}
    var helper func(n, k, i int)
    helper = func(n, k, i int) {
        if len(path) == k {
            tmp := make([]int, k)
            copy(tmp, path)
            res = append(res, tmp)
            return
        }
        if i > n {
            return
        }
        helper(n, k, i + 1)
        path = append(path, i)
        helper(n, k, i + 1)
        path = path[:len(path) - 1]
    }
    helper(n, k, 1)
    return res
}

代码随想录文章详解

理论基础
77.组合

总结

循环嵌入递归，循序渐进。感谢代码随想录训练营。