var singleNumber = function(nums) {
    const m = new Map();

    for(let v of nums) {
        if (m.has(v)) {
            m.set(v,m.get(v)+1);
        } else {
            m.set(v,1);
        }
    };
    for (let v of m) {
        if(v[1] === 1) {
            return v[0];
        }
    }
};