let str = 'B->A C->A D->B D->C D->E' //[A-B] [A-C] [B-D] [C-D] [E-D]
//[A,E][B,C][D]
let list = str.split(' ')
list = list.map(item=> item.split('->').reverse())
let result = [[]]
list.forEach( item=> {
let start = item[0]
let end = item[1]
let index1 = result.findIndex( item => { return item.includes(start)})
let index2 = result.findIndex( item => { return item.includes(end)})
//start和end都不存在
if(index1 == -1 && index2 == -1){
result[0].push(start)
if(!result[1]){
result[1] = []
}
result[1].push(end)
}else if( index1 == -1 && index2 > -1) { //start不存在 end存在
result[0].push(start)
}else if( index1 > -1 ) { //start存在
result = result.map( item => item.filter(el=> el!=end)) //去掉end 再插入end
if(!result[index1+1]){
result[index1+1] = []
}
result[index1+1].push(end)
}
})
let arr = []
result.forEach( item => {
arr = [...arr, ...item.sort()]; //同一层级默认字母排序
})
console.log(arr.join(' '))
