https://leetcode.com/problems/accounts-merge/submissions/

accountsMerge.js

/**
 * @param {string[][]} accounts
 * @return {string[][]}
 */
var accountsMerge = function(accounts) {
    
    const accountsWithEmail = new Map(); //<email: set(accountId)>
    accounts.forEach((account, id) => {
        for(let i=1; i<account.length; i++){
            const email = account[i];
            const name = account[0];
            if(!accountsWithEmail.get(email)) accountsWithEmail.set(email, new Set());
            accountsWithEmail.get(email).add(id);
        }
    });

    const merge = (id) => {
        const results = new Set();
        const stack = [id];
        const visited = new Set();
        
        while(stack.length){
            const currentAccountId = stack.pop();
            if(visited.has(currentAccountId)) continue;
            visited.add(currentAccountId);
            
            const account = accounts[currentAccountId];
            if(!account) continue;
            accounts[currentAccountId] = null; // else we'll "merge" every account for all IDs
            
            for(let i=1; i<account.length; i++){
                const email = account[i];
                results.add(email);
                const sisterAccounts = accountsWithEmail.get(email) || new Set();
                for(let sisterId of sisterAccounts) stack.push(sisterId);
            }
        }
        return results;
    };
    
    const results = [];
    for(let i=0; i<accounts.length; i++){
        const account = accounts[i];
        const id = i;
        if(!account) continue;
        const name = account[0];
        const emails = Array.from(merge(id)).sort();
        results.push([name].concat(emails));
    }
    
    return results;
};