[JS] Given list of births and deaths find year with most people alive

populationPeakYear.js

// Given list of births and deaths find year with most people alive (in case of tie return any)

function maxPopulation(yearsOfBirthNDeath) {
  const populationChange = []; 
  yearsOfBirthNDeath.forEach(([birth, death]) => {
    if(populationChange[birth]) populationChange[birth] += 1;
    else populationChange[birth] = 1;
    const yearAfterDeath = death + 1;
    if(populationChange[yearAfterDeath]) populationChange[yearAfterDeath] -= 1;
    else populationChange[yearAfterDeath] = -1;      
  })
  
  let maxPeople = 0; 
  let currentPeople = 0; 
  let yearWithMaxPeople = null; 
  populationChange.forEach( (populationDelta, year) => { 
    currentPeople += populationDelta; 
    if(currentPeople > maxPeople) {
      maxPeople = currentPeople;
      yearWithMaxPeople = year;
    }   
  });
  
  return [yearWithMaxPeople, maxPeople];
}


// ------------------------------------------- Test --------------------------------------------//
let years = [[2000, 2005], [2007, 2021], [2010, 2030], [2020,2021], [2020, 2036]];
// Each dotted sequence represents a person life span; we can see which years are the same by looking at starts and/or ends that align
// -----      --------
//            ---
//          -------
//        -------  
console.log(maxPopulation(years)); // [2020, 4]
years = [[2000, 2005], [2007, 2021]]; 
console.log(maxPopulationYear(years)); // [2000, 1]