從一個無序，不相等的數組中，選取N個數，使其和為M實現算法

// 遞歸分解，最后轉換成求2數之和
//  一個方法從 2Sum 秒殺到 100Sum
//  https://leetcode-cn.com/problems/3sum/solution/yi-ge-fang-fa-tuan-mie-by-labuladong/

var nSumTarget = function (nums, n, start, target) {
    let res = []
    if (n < 2 || n > nums.length) {
        return res
    }
    if (n == 2) {
        let low = start;
        let high = nums.length - 1;
        while (low < high) {
            let sum = nums[low] + nums[high];
            let left = nums[low]
            let right = nums[high]
            if (sum < target) {
                while (low < high && nums[low] === left) {
                    low++
                }
            } else if (sum > target) {
                while (low < high && nums[high] === right) {
                    high--
                }
            } else {
                res.push([left, right])
                while (low < high && nums[low] === left) {
                    low++
                }
                while (low < high && nums[high] === right) {
                    high--
                }
            }
        }
    } else {
        for (let i = start; i < nums.length; i++) {
            let sub = nSumTarget(nums, n - 1, i + 1, target - nums[i])
            for (let arr of sub) {
                arr.push(nums[i])
                res.push(arr)
            }
            while (i < nums.length && nums[i] === nums[i + 1]) {
                i++
            }
        }
    }
    return res
}
var findGroup = function (nums, n, sum) {
    nums = nums.sort((a, b) => a - b)
    return nSumTarget(nums, n, 0, sum)
};

// 位運算
// https://blog.csdn.net/weixin_34130269/article/details/91382220
const search = (arr, count, sum) => {
    // 計算某選擇情況下有幾個 `1`，也就是選擇元素的個數
    const n = num => {
        let count = 0
        while (num) {
            num &= (num - 1)
            count++
        }
        return count
    }

    let len = arr.length, bit = 1 << len, res = []

    // 遍歷所有的選擇情況
    for (let i = 1; i < bit; i++) {
        // 滿足選擇的元素個數 === count
        if (n(i) === count) {
            let s = 0, temp = []

            // 每一種滿足個數為 N 的選擇情況下，繼續判斷是否滿足 和為 M
            for (let j = 0; j < len; j++) {
                // 建立映射，找出選擇位上的元素
                if ((i & 1 << j) !== 0) {
                    s += arr[j]
                    temp.push(arr[j])

                    // 左移，從右邊的index往左邊看
                   // s += arr[len -1 - j]
                   // temp.push(arr[len -1 - j])
                }
            }

            // 如果這種選擇情況滿足和為 M
            if (s === sum) {
                res.push(temp)
            }
        }
    }

    return res
}

// 0,1背包問題 (沒有去重)
function find(arr, n, sum) {
    let res = []
    findGroup(arr, n, sum, [])
    function findGroup(arr, n, sum, oneRes) {
        if (n > arr.length) return false
        if (sum == 0 && n == 0) {
            res.push(oneRes)
            return true;
        } else if (n <= 0) {
            return false;
        }
        if (n > 0) {
            let temp = arr.slice(1, arr.length)
            findGroup(temp, n - 1, sum - arr[0], [...oneRes,arr[0]])
            findGroup(temp, n, sum, [...oneRes])
        }
    }
    return res
}

// 如果只要判斷能不能找到
function findGroup(arr, n, sum) {
    if (n > arr.length) return false
    if (sum == 0 && n == 0) {
        return true;
    } else if (n <= 0) {
        return false;
    }
    if (n > 0) {
        // if (arr.length === 0) return false
        let temp = arr.slice(1, arr.length)
        return findGroup(temp, n - 1, sum - arr[0]) || findGroup(temp, n, sum)
    }
}

參考資料：
https://wizardforcel.gitbooks.io/the-art-of-programming-by-july/content/02.03.html

https://blog.csdn.net/MrZZhou/article/details/77860278