https://leetcode.com/problems/evaluate-division/submissions/

evaluateDivision.js

/**
 * @param {string[][]} equations
 * @param {number[]} values
 * @param {string[][]} queries
 * @return {number[]}
 */
var calcEquation = function(equations, values, queries) {
    
    const idents = {}; // could use map but longer syntax
    
    equations.forEach((eq, i) => {
        const [numerator, denominator] = eq;
        idents[numerator] = idents[numerator] || [];
        idents[denominator] = idents[denominator] || [];
        idents[numerator].push([denominator, values[i], '*']);
        idents[denominator].push([numerator, values[i], '/']);
    });
    
    return queries.map(([n, d]) => {
        if(n === d && idents[n]) return 1;
        return find(n, d) || -1;
    });
    
    function find(n, d){ // finds n in terms of d. Returns number | null
        
        const seen = new Set();
        
        const traverse = (n, d) => {
            
            if(seen.has(`${n}:${d}`)) return null;
            else seen.add(`${n}:${d}`);
            
            if(idents[n]){
                for(let i=0; i< idents[n].length; i++){
                    const [variable, constant, operator] = idents[n][i];
                    if(variable === d) return operator === '*' ? constant : (1/constant);

                    const val = traverse(variable, d);
                    if(val !== null) return operator === '*' ? constant * val : val / constant;
                }
            }
            return null;    
        };
        
        return traverse(n, d);
    }
};