kingdomAndTrees.js

function kingdomAndTrees(a, maxX) {
  var lo = 1;
  var hi = maxX;
  var mid;


  while (lo < hi) {
    mid = Math.floor((hi + lo)/2); 
    var success = hasSolution(a, mid);
    
    if (success) {
      hi = mid;
    } else {
      lo = mid + 1;
    }
  }
  
  return lo;
}

function hasSolution(a, j) {
  var lastHeight = 0;
  var i;
  
  for (i = 0; i < a.length; i++) {
    if (a[i] + j > lastHeight) {
      lastHeight = Math.max(lastHeight + 1, a[i] - j);
    } else {
      return false;
    }
  }

  if (i === a.length) {
    return true;
  }
}