Add post-order traversal

DSA/post-order.cpp

@@ -0,0 +1,129 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+/**
+ * @brief Node structure for the binary tree.
+ */
+struct TreeNode {
+    int val;
+    TreeNode* left;
+    TreeNode* right;
+    TreeNode(int value) : val(value), left(nullptr), right(nullptr) {}
+};
+
+/**
+ * @brief Builds a binary tree level-wise from user input.
+ * 
+ * Input example:
+ * 1 2 3 4 5 -1 6 -1 -1 -1 -1 -1 -1
+ * (-1 means NULL)
+ */
+TreeNode* buildTreeLevelOrder() {
+    cout << "Enter nodes in level order (-1 for NULL):"<<endl;
+    string line;
+    getline(cin, line);
+    if (line.empty()) getline(cin, line);
+
+    stringstream ss(line);
+    vector<int> values;
+    int x;
+    while (ss >> x) values.push_back(x);
+
+    if (values.empty() || values[0] == -1) return nullptr;
+
+    TreeNode* root = new TreeNode(values[0]);
+    queue<TreeNode*> q;
+    q.push(root);
+    int i = 1;
+
+    while (!q.empty() && i < (int)values.size()) {
+        TreeNode* node = q.front();
+        q.pop();
+
+        // Left child
+        if (i < values.size() && values[i] != -1) {
+            node->left = new TreeNode(values[i]);
+            q.push(node->left);
+        }
+        i++;
+
+        // Right child
+        if (i < values.size() && values[i] != -1) {
+            node->right = new TreeNode(values[i]);
+            q.push(node->right);
+        }
+        i++;
+    }
+
+    return root;
+}
+
+/**
+ * @brief Recursive postorder traversal.
+ */
+void postorderRecursive(TreeNode* root, vector<int>& result) {
+    if (!root) return;
+    postorderRecursive(root->left, result);
+    postorderRecursive(root->right, result);
+    result.push_back(root->val);
+}
+
+/**
+ * @brief Iterative postorder traversal using two stacks.
+ */
+vector<int> postorderIterative(TreeNode* root) {
+    vector<int> result;
+    if (!root) return result;
+
+    stack<TreeNode*> s1, s2;
+    s1.push(root);
+
+    while (!s1.empty()) {
+        TreeNode* node = s1.top();
+        s1.pop();
+        s2.push(node);
+
+        if (node->left) s1.push(node->left);
+        if (node->right) s1.push(node->right);
+    }
+
+    while (!s2.empty()) {
+        result.push_back(s2.top()->val);
+        s2.pop();
+    }
+
+    return result;
+}
+
+/**
+ * @brief Utility function to print traversal results.
+ */
+void printTraversal(const vector<int>& traversal) {
+    for (int val : traversal) cout << val << " ";
+    cout << "\n";
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    TreeNode* root = buildTreeLevelOrder();
+
+    if (!root) {
+        cout << "Tree is empty."<<endl;
+        return 0;
+    }
+
+    // Recursive Postorder
+    vector<int> recursiveResult;
+    postorderRecursive(root, recursiveResult);
+    cout << "\nPostorder (Recursive): ";
+    printTraversal(recursiveResult);
+
+    // Iterative Postorder
+    vector<int> iterativeResult = postorderIterative(root);
+    cout << "Postorder (Iterative): ";
+    printTraversal(iterativeResult);
+
+    return 0;
+}