fix(composeExercise.js): keep same-Cangjie questions; make question a…

…dvancing O(1) time

The question 叭 (口金; rc) was inaccessible
due to its Cangjie code being the same as 只 (口金; rc).

This issue is fixed by using an Array for MapCharacterTable.questList.

* MapCharacterTable()
    * make this.questList an array (LIFO)
    * reverse the iteration of `this.questList`-building `for` loop
* MapCharacterTable.prototype.getCharacterAndAlphabet()
    * simplify question advancing by using Array.prototype.pop()
    * overwrite input table with the answer of the current question
      for handling characters with the same Cangjie code within the same stage

composeExercise.js

@@ -3,12 +3,12 @@
 function MapCharacterTable(characterUse) {
 	var characterArray = characterUse.textContent.split('\n');
 	var table = this.table = {};
-	var questList = this.questList = {}
-	for (var i=0, l=characterArray.length; i<l; i++) {
+	var questList = this.questList = [];
+	for (var i = characterArray.length - 1; i >= 0; --i) {
 		if (characterArray[i]) {
 			var character = characterArray[i].charAt(0), alphabet = characterArray[i].substr(1);
 			table[alphabet] = character;
-			questList[alphabet] = character;
+			questList.push([character, alphabet]);
 		}
 	}
 }
@@ -21,14 +21,9 @@ MapCharacterTable.prototype = {
 		return undefined;
 	},
 	getCharacterAndAlphabet: function (oldAlphabet) {
-		var list = this.questList;
-		list[oldAlphabet] = undefined;
-		for (var i in list) {
-			if (typeof(list[i]) == 'string' && list[i].length == 1) {
-				return [list[i], i];
-			}
-		}
-		return false;
+		var res = this.questList.pop();
+		this.table[res[1]] = res[0];
+		return res;
 	},
 	findCharacter: function (alphabet) {
 		return this.table[alphabet]