两个数之和 三个数之和 N个数之和

1. 两个数的和用map 当数组顺序的时候也可以使用二分查找

let nums = [2, 7, 11, 17],
    target = 9;
function twoSum(nums) {
    let map = new Map();
    let length = nums.length;
    for (let i = 0; i < length; i++) {
        if (map.has(target - nums[i])) {
            return [map.get(target - nums[i]), i];
        } else {
            map.set(nums[i], i);
        }
    }
    return -1;
}
let res = twoSum(nums);
console.log(res); //[ 0, 1 ]

2. 三个数的和为指定值target

需要先对数组arr进行排序，然后用set记录，

let arr = [1, 2, 7, 6, 2, 9, 0]
const threeSum = function(nums, target) {
    let result = [];
    nums.sort((a, b) => a - b);
    for (let i = 0; i < nums.length; i++) {
        while (nums[i] === nums[i - 1]) {
            i++;
        }
        let set = new Set();
        let first = nums[i];
        let j = i + 1;
        while (j < nums.length) {
            let second = target - nums[j] - first;
            if (set.has(second)) {
                result.push([first, second, nums[j]]);
                while (nums[j] === nums[j - 1]) {
                    j++;
                }
            }
            set.add(nums[j]);
            j++;
        }
    }
    return result;
}
let res = threeSum(arr, 10);
console.log(res); //[ [ 0, 1, 9 ], [ 1, 2, 7 ], [ 2, 2, 6 ] ]

3. 从数组中取出N个数和为指定数值

let arr = [1, 2, 3, 4];
let N = 3;
let M = 6;
const search = function(arr, count, sum) {
    const getCount = num => {
        let count = 0;
        while (num) {
            num &= (num - 1);
            count++;
        }
        return count;
    }
    let len = arr.length,
        bit = 1 << len,
        res = [];
    for (let i = 1; i < bit; i++) {
        if (getCount(i) === count) {
            let s = 0,
                temp = [];
            for (let j = 0; j < len; j++) {
                if (i & 1 << (len - 1 - j)) {
                    s += arr[j];
                    temp.push(arr[j]);
                }
            }
            if (s === sum) {
                res.push(temp)
            }
        }
    }
    return res;
}
let res = search(arr, N, M);
console.log(res);//[ [ 1, 2, 3 ] ]