modified solution 94 and 145

Author: Li Li

Algorithms Basics/Chapter 3. Binary Tree/145. Binary Tree Postorder Traversal.cs

@@ -67,24 +67,25 @@ public IList<int> PostorderTraversal(TreeNode root) {
         var stack = new Stack<TreeNode>();
         TreeNode lastVisit = null;
         while (root != null || stack.Count > 0) {
-            while (root != null) {
+            if (root != null) {
                 stack.Push(root);
                 root = root.left;
-            }
-            root = stack.Peek();
-            // if root.right == null, then root is leaf, just Add root
-            // if root.right == lastVisit, 
-            // then both left and right are visited, just Add root
-            if (root.right == null || root.right == lastVisit) {
-                res.Add(root.val);
-                // pop root, mark it as lastvisit, and let root = null,
-                // so we can keep going to peek stack top for next step
-                stack.Pop();
-                lastVisit = root;
-                root = null;
             } else {
-                // root.right not null, and it's not visited
-                root = root.right;
+                root = stack.Peek();
+                if (root.right != null || root.right != lastVisit) {
+                    // root.right not null, and it's not visited, keep going to right
+                    root = root.right;
+                } else {
+                    // if root.right == null, then root is leaf, just Add root
+                    // if root.right == lastVisit, 
+                    // then both left and right are visited, just Add root
+                    res.Add(root.val);
+                    // pop root, mark it as lastvisit, and let root = null,
+                    // so we can keep going to peek stack top for next step
+                    stack.Pop();
+                    lastVisit = root;
+                    root = null;
+                }
             }
         }
         return res;

Algorithms Basics/Chapter 3. Binary Tree/94. Binary Tree Inorder Traversal.cs

@@ -1,10 +1,4 @@
 // recursion
-/**
-* 利用栈实现循环先序遍历二叉树
-* 这种实现类似于图的深度优先遍历（DFS）
-* 维护一个栈，将根节点入栈，然后只要栈不为空，出栈并访问，接着依次将访问节点的右节点、左节点入栈。
-* 这种方式应该是对先序遍历的一种特殊实现（看上去简单明了），但是不具备很好的扩展性，在中序和后序方式中不适用
-*/ 
 public class Solution {
     public IList<int> InorderTraversal(TreeNode root) {
         var res = new List<int>();
@@ -20,28 +14,6 @@ public IList<int> InorderTraversal(TreeNode root) {
     }
 }
 
-/* 
-* 利用栈模拟递归过程实现循环先序遍历二叉树
-* 这种方式具备扩展性，它模拟递归的过程，将左子树点不断的压入栈，直到null，然后处理栈顶节点的右子树
-*/
-public class Solution {
-    public IList<int> PreorderTraversal(TreeNode root) {
-        var res = new List<int>();
-        if (root == null) return res;
-        Stack<TreeNode> stack = new Stack<TreeNode>();
-        while (root != null || stack.Count > 0) {
-            while (root != null) {
-                res.Add(root.val);
-                stack.Push(root);
-                root = root.left;
-            }
-            root = stack.Pop();
-            root = root.right;
-        }
-        return res;
-    }
-}
-
 /**
 * 
 * @param root 树根节点