clean up and add more notes

fishercoder1534 · fishercoder1534 · commit 087c1e4e943d · 2017-05-17T09:16:45.000-07:00
diff --git a/src/main/java/com/stevesun/solutions/_105.java b/src/main/java/com/stevesun/solutions/_105.java
@@ -11,24 +11,34 @@
  You may assume that duplicates do not exist in the tree.*/
 public class _105 {
 
-	//credit: https://discuss.leetcode.com/topic/29838/5ms-java-clean-solution-with-caching
-	//use HashMap as the cache so that accessing inorder index becomes O(1) time
+	/**credit: https://discuss.leetcode.com/topic/29838/5ms-java-clean-solution-with-caching
+	use HashMap as the cache so that accessing inorder index becomes O(1) time
+
+	 Note: The first element of preorder array is the root!*/
 	public TreeNode buildTree(int[] preorder, int[] inorder) {
 		Map<Integer, Integer> inorderMap = new HashMap();
 		for (int i = 0; i < inorder.length; i++) {
 			inorderMap.put(inorder[i], i);
 		}
 
+		/**At the beginning, both start from 0 to nums.length-1*/
 		return buildTree(preorder, 0, preorder.length-1, 0, inorder.length-1, inorderMap);
 	}
 
 	private TreeNode buildTree(int[] preorder, int preStart, int preEnd, int inStart, int inEnd, Map<Integer, Integer> inorderMap) {
 		if (preStart > preEnd || inStart > inEnd) return null;
 
 		TreeNode root = new TreeNode(preorder[preStart]);
-		int inRoot = inorderMap.get(root.val);
+		int inRoot = inorderMap.get(preorder[preStart]);
 		int numsLeft = inRoot - inStart;
 
+		/**It's easy to understand and remember:
+		 * for the indices of inorder array:
+		 * root.left should be inStart and inRoot-1 as new start and end indices
+		 * root.right should be inRoot+1 and inEnd as new start and end indices
+		 *
+		 * since inRoot is being used already in this recursion call, that's why we use inRoot-1 and inRoot+1
+		 * this part is the same for both Leetcode 105 and Leetcode 106.*/
 		root.left = buildTree(preorder, preStart+1, preStart+numsLeft, inStart, inRoot-1, inorderMap);
 		root.right = buildTree(preorder, preStart+numsLeft+1, preEnd, inRoot+1, inEnd, inorderMap);
 		return root;
diff --git a/src/main/java/com/stevesun/solutions/_106.java b/src/main/java/com/stevesun/solutions/_106.java
@@ -16,8 +16,11 @@
  */
 public class _106 {
 
-    //credit: https://discuss.leetcode.com/topic/3296/my-recursive-java-code-with-o-n-time-and-o-n-space
-    /**The idea is to take the last element in postorder as the root; find the position of the root in the inorder array;
+    /**https://discuss.leetcode.com/topic/3296/my-recursive-java-code-with-o-n-time-and-o-n-space
+     *
+     * Note: the last element of postorder array is the root!
+     *
+     * The idea is to take the last element in postorder as the root; find the position of the root in the inorder array;
      * then locate the range for left sub-tree and right sub-tree and do recursion,
      * use a hashmap to record the index of root in the inorder array.*/
     public TreeNode buildTree(int[] inorder, int[] postorder) {
@@ -28,17 +31,32 @@ public TreeNode buildTree(int[] inorder, int[] postorder) {
         for (int i = 0; i < inorder.length; i++) {
             inorderMap.put(inorder[i], i);
         }
+        /**At the beginning, both start from 0 to nums.length-1*/
         return buildTreeRecursively(0, inorder.length-1, postorder, 0, postorder.length-1,inorderMap);
     }
 
     private TreeNode buildTreeRecursively(int inorderStart, int inorderEnd, int[] postorder, int postorderStart, int postorderEnd, Map<Integer, Integer> inorderMap){
         if (postorderStart > postorderEnd || inorderStart > inorderEnd) return null;
         TreeNode root = new TreeNode(postorder[postorderEnd]);
         int inRoot = inorderMap.get(postorder[postorderEnd]);
-        TreeNode leftchild = buildTreeRecursively(inorderStart, inRoot-1, postorder, postorderStart, postorderStart+inRoot-inorderStart-1, inorderMap);
-        TreeNode rightchild = buildTreeRecursively(inRoot+1, inorderEnd, postorder, postorderStart+inRoot-inorderStart, postorderEnd-1, inorderMap);
-        root.left = leftchild;
-        root.right = rightchild;
+        int numsLeft = inRoot-inorderStart;
+
+        /**It's easy to understand and remember:
+         * for the indices of inorder array:
+         * inStart and inRoot-1 as new start and end indices
+         * inRoot+1 and inEnd as new start and end indices
+         *
+         * this is easy to understand and remember: since inRoot is already been used in this recursion call, so we're going to use inRoot-1 and inRoot+1 for next recursion call
+         *
+         * for the indices of postorder array:
+         * postorderStart and postorderStart+numsLeft-1 should be the new start and end indices
+         * postorderStart+numsLeft and postorderEnd-1 should be the new start and end indices
+         *
+         * this is also easy to understand and remember:
+         * since the last one in postorder is the root and we have used it in this recursion call already, so the end is definitely postorderEnd-1;
+         * then the postorderEnd for root.left is contiguous to the postorderStart of root.right, :)*/
+        root.left = buildTreeRecursively(inorderStart, inRoot-1, postorder, postorderStart, postorderStart+numsLeft-1, inorderMap);
+        root.right = buildTreeRecursively(inRoot+1, inorderEnd, postorder, postorderStart+numsLeft, postorderEnd-1, inorderMap);
         return root;
     }
 
