2021-07-28：最短的桥。在给定的二维二进制数组 A 中，存

package main

import (
    "fmt"
    "math"
)

func main() {
    m := [][]int{{0, 1, 0}, {0, 0, 0}, {0, 0, 1}}
    ret := shortestBridge(m)
    fmt.Println(ret)
}

func shortestBridge(m [][]int) int {
    N := len(m)
    M := len(m[0])
    all := N * M
    island := 0
    curs := make([]int, all)
    nexts := make([]int, all)
    records := make([][]int, 2)
    for i := 0; i < 2; i++ {
        records[i] = make([]int, all)
    }
    for i := 0; i < N; i++ {
        for j := 0; j < M; j++ {
            if m[i][j] == 1 { // 当前位置发现了1！
                // 把这一片的1，都变成2，同时，抓上来了，这一片1组成的初始队列
                // curs, 把这一片的1到自己的距离，都设置成1了，records
                queueSize := infect(m, i, j, N, M, curs, 0, records[island])
                V := 1
                for queueSize != 0 {
                    V++
                    // curs里面的点，上下左右，records[点]==0， nexts
                    queueSize = bfs(N, M, all, V, curs, queueSize, nexts, records[island])
                    tmp := curs
                    curs = nexts
                    nexts = tmp
                }
                island++
            }
        }
    }
    min := math.MaxInt64
    for i := 0; i < all; i++ {
        min = getMin(min, records[0][i]+records[1][i])
    }
    return min - 3
}

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

// 当前来到m[i][j] , 总行数是N，总列数是M
// m[i][j]感染出去(找到这一片岛所有的1),把每一个1的坐标，放入到int[] curs队列！
// 1 (a,b) -> curs[index++] = (a * M + b)
// 1 (c,d) -> curs[index++] = (c * M + d)
// 二维已经变成一维了， 1 (a,b) -> a * M + b
// 设置距离record[a * M +b ] = 1
func infect(m [][]int, i int, j int, N int, M int, curs []int, index int, record []int) int {
    if i < 0 || i == N || j < 0 || j == M || m[i][j] != 1 {
        return index
    }
    // m[i][j] 不越界，且m[i][j] == 1
    m[i][j] = 2
    p := i*M + j
    record[p] = 1
    // 收集到不同的1
    curs[index] = p
    index++
    index = infect(m, i-1, j, N, M, curs, index, record)
    index = infect(m, i+1, j, N, M, curs, index, record)
    index = infect(m, i, j-1, N, M, curs, index, record)
    index = infect(m, i, j+1, N, M, curs, index, record)
    return index
}

// 二维原始矩阵中，N总行数，M总列数
// all 总 all = N * M
// V 要生成的是第几层 curs V-1 nexts V
// record里面拿距离
func bfs(N int, M int, all int, V int, curs []int, size int, nexts []int, record []int) int {
    nexti := 0 // 我要生成的下一层队列成长到哪了？
    for i := 0; i < size; i++ {
        // curs[i] -> 一个位置
        up := twoSelectOne(curs[i] < M, -1, curs[i]-M)
        down := twoSelectOne(curs[i]+M >= all, -1, curs[i]+M)
        left := twoSelectOne(curs[i]%M == 0, -1, curs[i]-1)
        right := twoSelectOne(curs[i]%M == M-1, -1, curs[i]+1)
        if up != -1 && record[up] == 0 {
            record[up] = V
            nexts[nexti] = up
            nexti++
        }
        if down != -1 && record[down] == 0 {
            record[down] = V
            nexts[nexti] = down
            nexti++
        }
        if left != -1 && record[left] == 0 {
            record[left] = V
            nexts[nexti] = left
            nexti++
        }
        if right != -1 && record[right] == 0 {
            record[right] = V
            nexts[nexti] = right
            nexti++
        }
    }
    return nexti
}

func twoSelectOne(c bool, a int, b int) int {
    if c {
        return a
    } else {
        return b
    }
}