m选n组合

最近需求，要写排列组合算法，首先第一步是m个元素中选n个元素进行组合，也就是数学中C(m,n)；方法有多种。

递归

let arr = [1,2,3]
function combination(arr, nLen, m, singleArr, list) {
    if (m === 0) {
        let arrCopy = []
        for (let j = 0; j < singleArr.length; j++) {
            arrCopy[j] = singleArr[j]
        }
        list.push(arrCopy)
        return
    }

    for (let i = nLen; i >= m; --i) {
        singleArr[m - 1] = arr[i - 1]
        combination(arr, i - 1, m - 1, singleArr, list)
    }
}

function getArrlist (arr, num) {
    let singleArr = []
    let list = []
    combination(arr, arr.length, num, singleArr, list)
    return list
}

let list = getArrlist(arr, 2) // [2,3] [1,3] [1,2]

01法