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POHow to improve runtime of a recursive function
 

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  1. POHow to improve runtime of a recursive function
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    <p>In solving this problem: <a href="http://www.cs.duke.edu/csed/newapt/drawtree.html" rel="nofollow">http://www.cs.duke.edu/csed/newapt/drawtree.html</a> I wrote the code below but it seems to run too slow. Is there a faster way of checking all of the child nodes w/o using a FOR loop? Would a queue be helpful? </p> <pre><code>public class DrawTree { HashMap&lt;String, ArrayList&lt;String&gt;&gt; map = new HashMap&lt;String, ArrayList&lt;String&gt;&gt;(); ArrayList&lt;String&gt; drawing = new ArrayList&lt;String&gt;(); String root; public String[] draw(int[] parents, String[] names) { for(int x=0; x&lt;parents.length; x++) { int parentindex = parents[x]; //root name if(parentindex==-1) { root=names[x]; if(!map.containsKey(names[x])) { map.put(names[x], new ArrayList&lt;String&gt;()); } continue; } //add parent, child to map if (!map.containsKey(names[parentindex])) map.put(names[parentindex], new ArrayList&lt;String&gt;()); map.get(names[parentindex]).add(names[x]); } sketch("",root,false); return drawing.toArray(new String[drawing.size()]); } //***IMPROVE RUN TIME - different algorithm??*** //method takes root and prefix? public void sketch(String parent, String child, boolean addPipe){ StringBuilder toAdd = new StringBuilder(); //don't need to add connector pipe if(!addPipe) { //number of spaces to add to prefix int spaces = parent.indexOf('-')+1; //add spaces to prefix while(spaces&gt;0) { toAdd.append(" "); spaces--; } toAdd.append("+-"+child); } //index of pipe in parent, -1 if parent doesn't have pipe int parentPipe = parent.indexOf('|'); //need to add connector pipe &amp; parent has pipe // (is a child of a subtree) if(parentPipe&gt;0) { //number of spaces to add to prefix int spaces = parent.indexOf('-')+1; //add spaces to prefix while(spaces&gt;0) { if(spaces==parentPipe) toAdd.append('|'); else toAdd.append(" "); spaces--; } toAdd.append("+-"+child); } //need to add pipe and parent doesn't have pipe if(addPipe &amp;&amp; parentPipe&lt;0) { int spaces = parent.indexOf('-')+1; while(spaces&gt;0) { if(spaces==2) toAdd.append('|'); else toAdd.append(" "); } toAdd.append("+-"+child); } //add child to list of tree drawing String node = toAdd.toString(); drawing.add(node); //System.out.println(node); //loop through list of children, passing each recursively //...count level? if(map.containsKey(child)) { //System.out.println("map works"); for(int x = 0; x&lt;map.get(child).size(); x++) { boolean pipe = false; if(x&lt;(map.get(child).size()-1)) pipe=true; //System.out.println(map.get(child).get(x)); sketch(node, map.get(child).get(x), pipe); } } } </code></pre>
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    1. PTQuestion
    1. USStephen C
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    1. COThis question is a better fit for [codereview.se](http://codereview.stackexchange.com/).
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      1. UScheeken
    2. COAnd what is the basis for defining as "slow"? Do you have any particular timings in mind?
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      1. USkosa
    3. COOne improvement is for map.containsKey(key). It has almost the same time like map.get(key). It has no sense to call containsKey and get one by one. Better use `List<String> list = map.get(key);if(list==null) {list=new ArrayList<String>(); map.put(key,list);} list.add(something);`
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      1. USAndrzej Jozwik
 

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