add code2 of iterative solution

solutions/94. Binary Tree Inorder Traversal.md

@@ -30,27 +30,53 @@ public:
 ```
 
 ## 思路二
+### 写法一
 ``` C++
 class Solution {
 public:
     vector<int> inorderTraversal(TreeNode* root) {
         vector<int>res;
-        if(!root) return res;
         stack<TreeNode *>stk;
         TreeNode* p = root;        
         while(!stk.empty() || p){
-            if(p){
+            if(p){ // 左: 一路向左下降
                 stk.push(p);
-                p = p -> left; // 一路向左
+                p = p -> left; 
             }
             else{
-                p = stk.top();
+                p = stk.top(); // 中: 下降不动了, 访问栈顶元素
                 stk.pop();
                 res.push_back(p -> val);
-                p = p -> right;
+                p = p -> right; // 右: 访问右子树
             }
         }
         return res;
     }
 };
 ```
+### 写法二
+``` C++
+class Solution {
+public:
+    vector<int> inorderTraversal(TreeNode* root) {
+        vector<int>res;
+        stack<TreeNode*>stk;
+        TreeNode *p = root;
+        
+        while(!stk.empty() || p){
+            // 左: 一路向左下降
+            while(p){ 
+                stk.push(p);
+                p = p -> left; 
+            }
+            // 中: 下降不动了, 访问栈顶元素
+            p = stk.top(); stk.pop();
+            res.push_back(p -> val);
+            // 右: 访问右子树
+            p = p -> right; 
+        }
+        
+        return res;
+    }
+};
+```