2021-07-30：两个有序数组间相加和的Topk问题。给定两个有序数组arr1和arr2，再给定一个整数k，返回来自arr1

package main

import (
    "fmt"
    "sort"
)

func main() {
    arr1 := []int{1, 2, 3, 4, 5}
    arr2 := []int{3, 5, 7, 9, 11}
    topK := 4
    if true {
        ret := topKSum1(arr1, arr2, topK)
        fmt.Println("左神的方法：", ret)
    }
    if true {
        ret := topKSum2(arr1, arr2, topK)
        fmt.Println("我的方法：", ret)
    }
}

type Node struct {
    index1 int // arr1中的位置
    index2 int // arr2中的位置
    sum    int // arr1[index1] + arr2[index2]的值
}

func NewNode(i1 int, i2 int, s int) *Node {
    ret := &Node{}
    ret.index1 = i1
    ret.index2 = i2
    ret.sum = s
    return ret
}

func topKSum1(arr1 []int, arr2 []int, topK int) []int {
    if len(arr1) == 0 || len(arr2) == 0 || topK < 1 {
        return nil
    }
    N := len(arr1)
    M := len(arr2)
    topK = getMin(topK, N*M)
    res := make([]int, topK)
    resIndex := 0
    maxHeap := make([]*Node, 0)
    set := make(map[int]struct{})
    i1 := N - 1
    i2 := M - 1
    Push(&maxHeap, NewNode(i1, i2, arr1[i1]+arr2[i2]))
    set[x(i1, i2, M)] = struct{}{}
    for resIndex != topK {
        curNode := Pop(&maxHeap)
        res[resIndex] = curNode.sum
        resIndex++
        i1 = curNode.index1
        i2 = curNode.index2
        delete(set, x(i1, i2, M))
        _, ok := set[x(i1-1, i2, M)]
        if i1-1 >= 0 && !ok {
            set[x(i1-1, i2, M)] = struct{}{}
            Push(&maxHeap, NewNode(i1-1, i2, arr1[i1-1]+arr2[i2]))
        }
        _, ok = set[x(i1, i2-1, M)]
        if i2-1 >= 0 && !ok {
            set[x(i1, i2-1, M)] = struct{}{}
            Push(&maxHeap, NewNode(i1, i2-1, arr1[i1]+arr2[i2-1]))
        }
    }
    return res
}

func getMin(a int, b int) int {
    if a < b {
        return a
    } else {
        return b
    }
}

func x(i1 int, i2 int, M int) int {
    return i1*M + i2
}

func Push(maxHeap *[]*Node, node *Node) {
    *maxHeap = append(*maxHeap, node)
}

func Pop(maxHeap *[]*Node) *Node {
    sort.Slice(*maxHeap, func(i, j int) bool {
        return (*maxHeap)[i].sum < (*maxHeap)[j].sum
    })
    ans := (*maxHeap)[len(*maxHeap)-1]
    *maxHeap = (*maxHeap)[0 : len(*maxHeap)-1]
    return ans
}

func topKSum2(arr1 []int, arr2 []int, topK int) []int {
    if len(arr1) == 0 || len(arr2) == 0 || topK < 1 {
        return nil
    }
    N := len(arr1)
    M := len(arr2)
    topK = getMin(topK, N*M)
    maxHeap := make([]int, 0)
    i1Start, i2Start := N-1, M-1
    count := 0
    for {
        for i1, i2 := i1Start, i2Start; i1 >= 0 && i2 < M; i1, i2 = i1-1, i2+1 {
            PushInt(&maxHeap, arr1[i1]+arr2[i2])
            if len(maxHeap) > topK {
                PopInt(&maxHeap)
            }
            count++
        }
        if count >= topK {
            break
        }
        if i2Start > 0 { //左移
            i2Start--
        } else { //上移
            i1Start--
        }

    }

    return maxHeap
}

func PushInt(maxHeap *[]int, node int) {
    *maxHeap = append(*maxHeap, node)
}

func PopInt(maxHeap *[]int) int {
    sort.Slice(*maxHeap, func(i, j int) bool {
        return (*maxHeap)[i] > (*maxHeap)[j]
    })
    ans := (*maxHeap)[len(*maxHeap)-1]
    *maxHeap = (*maxHeap)[0 : len(*maxHeap)-1]
    return ans
}