Help travellers get from one side of a bridge to another with only a lantern to guide them. (Solution is random/brute force, not exhaustive/optimal.)

ZombieBridge.js

// Set up initial constraints.
GOAL_TOTAL_TIME = parseInt(process.argv.slice(2)[1]); // if optimal time unknown, set 0.
MAX_ATTEMPTS = parseInt(process.argv.slice(2)[2]); // I don't recommend going past 50000.
try { INITIAL_AT_RISK_PEOPLE = JSON.parse(process.argv.slice(2)[0]); } finally {
  if (typeof INITIAL_AT_RISK_PEOPLE === 'undefined' || isNaN(GOAL_TOTAL_TIME) || isNaN(MAX_ATTEMPTS) || !Array.isArray(INITIAL_AT_RISK_PEOPLE)) {
    console.log("\nInvalid command format. The command should look like this (space-sensitive):\n");
    console.log("  node ZombieBridge.js [1,2,5,10] 17 1000\n");
    return; // Return early since the command won't work.
  } else console.log("INITIAL CONSTRAINTS:", {INITIAL_AT_RISK_PEOPLE,GOAL_TOTAL_TIME, MAX_ATTEMPTS}, "\n");
}

// Set up variables to track the best solution we find as well as how many attempts we've tried.
attemptNumber = 0;
bestSolutionSteps = [];
bestTotalTimeFound = Infinity // Will decrease over time as better solutions are found.

while (bestTotalTimeFound > GOAL_TOTAL_TIME && attemptNumber++ < MAX_ATTEMPTS) {
  // Whe starting a new attempt we reset the time and steps and we move people and the lantern back
  // to their starting positions.
  timeUsed = 0;
  stepsUsed = []
  atRiskPeople = [...INITIAL_AT_RISK_PEOPLE];
  safePeople = [];
  isLanternSafe = false;

  // Keep doing the following until everybody is safe.
  while(atRiskPeople.length > 0) {
    // If the lantern is on the "safe" side, pick one person to walk back to the "at risk" side,
    // otherwise pick two people to walk from the "at risk" side to the "safe" side.
    walkers = isLanternSafe ? pick(1, safePeople) : pick(2, atRiskPeople);
    // Add the time of the slowest walker to the total time for this solution.
    timeUsed = timeUsed + max(walkers);

    // Store the text representing the step (and output it) so in case it's the best solution we
    // can show it at the end.
    stepsUsed.push(`${pad("#"+attemptNumber, "#"+MAX_ATTEMPTS)} @ ${pad(timeUsed, GOAL_TOTAL_TIME || "999")}min: \t ${pad(atRiskPeople, INITIAL_AT_RISK_PEOPLE)} \t AT RISK \t ${isLanternSafe ? '<' : '-'}-${pad(walkers, INITIAL_AT_RISK_PEOPLE, "-")}-${isLanternSafe ? '-' : '>'} \t SAFE \t ${safePeople}`);
    console.log(stepsUsed[stepsUsed.length-1]);

    // Move the walker(s) and the lantern to their new side.
    walkers.forEach(walker => (isLanternSafe ? atRiskPeople : safePeople).push(walker));
    isLanternSafe = !isLanternSafe;
  }

  // If this solution was better than the best solution we've found so far, save this one as the
  // new best solution.
  if (timeUsed < bestTotalTimeFound) {
    bestTotalTimeFound = timeUsed;
    bestSolutionSteps = stepsUsed;
  }
  console.log(""); // Add a carriage return/new line to visually separate each solution's steps.
}

// Now that we've either achieved the goal time or run out of attempts, output the best solution.
console.log("BEST SOLUTION FOUND:\n" + bestSolutionSteps.join("\n"));

// These are utility functions. Note that pick chooses n items _at random_.
function max (pair) { return Math.max(...pair); }
function pick (howMany, toRemoveFrom) { var results = []; while (results.length < howMany) { var indexToRemove = Math.floor(Math.random()*toRemoveFrom.length); var itemToRemove = toRemoveFrom[indexToRemove]; toRemoveFrom.splice(indexToRemove,1); results.push(itemToRemove); } return results; }
function pad(string, maxStr, char=" ") { while (String(string).length < String(maxStr).length) string = char + string; return string; }

Example output from running node ZombieBridge.js [1,2,5,10] 17 1000: (also runnable online here.)

    #1 @  5min: 	     1,10 	 AT RISK 	 -------2,5-> 	 SAFE 	 
    #1 @ 10min: 	     1,10 	 AT RISK 	 <--------5-- 	 SAFE 	 2
    #1 @ 20min: 	        1 	 AT RISK 	 ------10,5-> 	 SAFE 	 2
    #1 @ 25min: 	        1 	 AT RISK 	 <--------5-- 	 SAFE 	 2,10
    #1 @ 30min: 	          	 AT RISK 	 -------5,1-> 	 SAFE 	 2,10

    #2 @ 10min: 	      1,2 	 AT RISK 	 ------10,5-> 	 SAFE 	 
    #2 @ 20min: 	      1,2 	 AT RISK 	 <-------10-- 	 SAFE 	 5
    #2 @ 30min: 	        1 	 AT RISK 	 ------10,2-> 	 SAFE 	 5
    #2 @ 32min: 	        1 	 AT RISK 	 <--------2-- 	 SAFE 	 5,10
    #2 @ 34min: 	          	 AT RISK 	 -------1,2-> 	 SAFE 	 5,10

    #3 @  2min: 	     5,10 	 AT RISK 	 -------2,1-> 	 SAFE 	 
    #3 @  4min: 	     5,10 	 AT RISK 	 <--------2-- 	 SAFE 	 1
    #3 @ 14min: 	        2 	 AT RISK 	 ------10,5-> 	 SAFE 	 1
    #3 @ 15min: 	        2 	 AT RISK 	 <--------1-- 	 SAFE 	 10,5
    #3 @ 17min: 	          	 AT RISK 	 -------2,1-> 	 SAFE 	 10,5

bestSolution:
    #3 @  2min: 	     5,10 	 AT RISK 	 -------2,1-> 	 SAFE 	 
    #3 @  4min: 	     5,10 	 AT RISK 	 <--------2-- 	 SAFE 	 1
    #3 @ 14min: 	        2 	 AT RISK 	 ------10,5-> 	 SAFE 	 1
    #3 @ 15min: 	        2 	 AT RISK 	 <--------1-- 	 SAFE 	 10,5
    #3 @ 17min: 	          	 AT RISK 	 -------2,1-> 	 SAFE 	 10,5