hash表
function duplicates(arr) { const target = []; const map = new Map(); arr.forEach(v => { if(map.has(v) && !target.includes(v)) { target.push(v); } else { map.set(v, v); } }); return target; }
function duplicates(arr) { const target = []; const map = new Map(); arr.forEach(v => { if(map.has(v) && !target.includes(v)) { target.push(v); } else { map.set(v, v); } }); return target; }