Implement parsing a postfix expression into a tree.

src/org/nwapw/abacus/tree/NumberNode.java

@@ -0,0 +1,25 @@
+package org.nwapw.abacus.tree;
+
+import org.nwapw.abacus.number.NaiveNumber;
+import org.nwapw.abacus.number.Number;
+
+public class NumberNode extends TreeNode {
+
+    private Number number;
+
+    public NumberNode(){
+        number = null;
+    }
+
+    public NumberNode(double value){
+        number = new NaiveNumber(value);
+    }
+
+    public NumberNode(String value){
+        this(Double.parseDouble(value));
+    }
+
+    public Number getNumber() {
+        return number;
+    }
+}

src/org/nwapw/abacus/tree/OpNode.java

@@ -0,0 +1,38 @@
+package org.nwapw.abacus.tree;
+
+public class OpNode extends TreeNode {
+
+    private String operation;
+    private TreeNode left;
+    private TreeNode right;
+
+    public OpNode(String operation){
+        this(operation, null, null);
+    }
+
+    public OpNode(String operation, TreeNode left, TreeNode right){
+        this.operation = operation;
+        this.left = left;
+        this.right = right;
+    }
+
+    public String getOperation() {
+        return operation;
+    }
+
+    public TreeNode getLeft() {
+        return left;
+    }
+
+    public void setLeft(TreeNode left) {
+        this.left = left;
+    }
+
+    public TreeNode getRight() {
+        return right;
+    }
+
+    public void setRight(TreeNode right) {
+        this.right = right;
+    }
+}

src/org/nwapw/abacus/tree/TreeNode.java

@@ -78,8 +78,26 @@ public abstract class TreeNode {
         return output;
     }
 
-    public static TreeNode fromString(String string){
+    public static TreeNode fromStringRecursive(String source, ArrayList<Match<TokenType>> matches){
+        if(matches.size() == 0) return null;
+        Match<TokenType> match = matches.remove(0);
+        if(match.getType() == TokenType.OP){
+            TreeNode right = fromStringRecursive(source, matches);
+            TreeNode left = fromStringRecursive(source, matches);
+            if(left == null || right == null) return null;
+            else return new OpNode(source.substring(match.getFrom(), match.getTo()), left, right);
+        } else if(match.getType() == TokenType.NUM){
+            return new NumberNode(Double.parseDouble(source.substring(match.getFrom(), match.getTo())));
+        }
         return null;
     }
 
+    public static TreeNode fromString(String string){
+        ArrayList<Match<TokenType>> matches = intoPostfix(string, tokenize(string));
+        if(matches == null) return null;
+
+        Collections.reverse(matches);
+        return fromStringRecursive(string, matches);
+    }
+
 }