Complete characterFrequency.js in O(n*log(n)) time complexity

characterFrequency/characterFrequency.js

@@ -27,7 +27,100 @@
  *
  */
 
+// O(n*log(n)) + O(n) + O(n) => O(n*log(n))
+var characterFrequency = function (string) {
+  var freq   = {};
+  var result = [];
+  var length = string.length;
+  var i, prop, temp;
+
+  // O(n)
+  for (i = 0; i < length; i += 1) {
+    if (freq[string[i]] === void 0) {
+      freq[string[i]] = 1;
+    } else {
+      freq[string[i]] += 1;
+    }
+  }
+
+  // freq is an object with all the frequencies for each character
+  // O(n)
+  for (prop in freq) {
+    result.push( [prop, freq[prop]] );
+  }
+
+  // result is an array of [character, frequency]'s
+  // O(n*log(n))
+  mergeSort(result);
 
-var characterFrequency = function(string) {
   return result;
 };
+
+// O(n*log(n))
+var mergeSort = function (array) {
+  var length = array.length;
+
+  if (length < 2) { return array; }
+
+  var mid   = Math.floor( length / 2 );
+  var left  = array.slice(0, mid);
+  var right = array.slice(mid);
+
+  left  = mergeSort(left);
+  right = mergeSort(right);
+  merge(array, left, right);
+
+  return array;
+};
+
+var merge = function (array, left, right) {
+  // each element in any of the arrays is an array of [character, frequency]
+  var a    = 0;
+  var l    = 0;
+  var r    = 0;
+  var aLen = array.length;
+  var lLen = left.length;
+  var rLen = right.length;
+
+  while (l < lLen && r < rLen) {
+    if (left[l][1] < right[r][1]) {
+      // descending order
+      array[a] = right[r];
+      a += 1;
+      r += 1;
+    } else if (left[l][1] > right[r][1]) {
+      array[a] = left[l];
+      a += 1;
+      l += 1;
+    } else {
+      // when the frequencies are equal
+      if ( getCharCode(left[l][0]) < getCharCode(right[r][0]) ) {
+        array[a] = left[l];
+        a += 1;
+        l += 1;
+      } else {
+        array[a] = right[r];
+        a += 1;
+        r += 1;
+      }
+    }
+  }
+
+  // by the time the above while loop terminates, either left or right arrays
+  // is still not exhausted
+  while (l < lLen) {
+    array[a] = left[l];
+    a += 1;
+    l += 1;
+  }
+
+  while (r < rLen) {
+    array[a] = right[r];
+    a += 1;
+    r += 1;
+  }
+};
+
+var getCharCode = function (character) {
+  return String.prototype.charCodeAt.call(character);
+};