Array Merge without Duplicates
JavaScript O(N) 131 124 116 92 (86?)

Golfed version:

function m(i,x){h={};n=[];for(a=2;a--;i=x)i.map(function(b){h[b]=h[b]||n.push(b)});return n}


Human readable golfed version:

function m(i,x) {
   h = {}
   n = []
   for (a = 2; a--; i=x)
      i.map(function(b){
        h[b] = h[b] || n.push(b)
      })
   return n
}


I could use concat like so and do it in 86 characters:

function m(i,x){h={};n=[];i.concat(x).map(function(b){h[b]=h[b]||n.push(b)});return n}


But I am not sure if it is still O(N) based on this JsPerf: http://jsperf.com/unique-array-merging-concat-vs-looping as the concat version is marginally faster with smaller arrays but slower with larger arrays (Chrome 31 OSX).

In practice do this (golf is full of bad practices):

function merge(a1, a2) {
   var hash = {};
   var arr = [];
   for (var i = 0; i &lt; a1.length; i++) {
      if (hash[a1[i]] !== true) {
        hash[a1[i]] = true;
        arr[arr.length] = a1[i];
      }
   }
   for (var i = 0; i &lt; a2.length; i++) {
      if (hash[a2[i]] !== true) {
        hash[a2[i]] = true;
        arr[arr.length] = a2[i];
      }
   }
   return arr;
}
console.log(merge([1,2,3,4,5],[1,2,3,4,5,6]));


I'm not great at computing complexity but I believe this is O(N). Would love if someone could clarify.

Edit: Here is a version that takes any number of arrays and merges them.

function merge() {
   var args = arguments;
   var hash = {};
   var arr = [];
   for (var i = 0; i &lt; args.length; i++) {
      for (var j = 0; j &lt; args[i].length; j++) {
        if (hash[args[i][j]] !== true) {
          arr[arr.length] = args[i][j];
          hash[args[i][j]] = true;
        }
      }
    }
   return arr;
}
console.log(merge([1,2,3,4,5],[1,2,3,4,5,6],[1,2,3,4,5,6,7],[1,2,3,4,5,6,7,8]));