// Using map --> create map // key--> item, value --> occurences of item in input array (a) // iterate through map --> return key whose value is 1

let numMap = new Map()

a.forEach(item=> {
    if(numMap.has(item)){
        numMap.set(item,numMap.get(item)+1)
    } else {
        numMap.set(item,1)
    }
})

for (let [key,value] of numMap) {
    if(value===1) { return key}
}