vault backup: 2022-07-04 15:32:45

Leetcode Binary-Tree-Preorder-Traversal.md

@@ -59,7 +59,52 @@ Preorder, means root is at the "Pre" position, so the order is:
 
 The recursion version is easy to implement.
 
+Use Stacks for the iteration method, because stacks are FILO, the **subtree will get traversed first.**
+And remember to push **right subtree** first, so that the left one will be traversed first.
+
+
 ### Solution
+Iteration
+```cpp
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
+ * right(right) {}
+ * };
+ */
+class Solution {
+public:
+  vector<int> preorderTraversal(TreeNode *root) {
+    if (!root) {
+      return {};
+    }
+
+    // Using stacks, since FILO, the latest gets served first.
+    vector<int> answer;
+    stack<TreeNode *> pending;
+
+    do {
+      if (root != nullptr) {
+        answer.push_back(root->val);
+        // nothing gets pushed if is nullptr;
+        pending.push(root->right);
+        pending.push(root->left);
+      }
+      root = pending.top();
+      pending.pop();
+    } while (root || !pending.empty());
+
+    return answer;
+  }
+};
+
+```
 Recursion, using private funcs:
 ```cpp
 /**