added creating tree when traversals are given

Author: Sai-Adithya-M

data_structures/create_tree_from_traversals.cpp

@@ -0,0 +1,98 @@
+#include <iostream>
+using namespace std;
+#include <map>
+#include <stack>
+#include <vector>
+
+class node {
+ public:
+    int data;
+    node* left;
+    node* right;
+    node* parent;
+    node(int n) {
+        data = n;
+        left = NULL;
+        right = NULL;
+        parent = NULL;
+    }
+};
+//normal inorder traversal
+void inorder(node* head, vector<int>& out) {
+    node* cur = head;
+    if (!cur)
+        return;
+    inorder(cur->left, out);
+    out.push_back(cur->data);
+    inorder(cur->right, out);
+}
+
+//when preorder and inorder vectors are given
+node* buildtree2pre(vector<int>& preorder, int preStart, int preEnd,
+                    vector<int>& inorder, int inStart, int inEnd,
+                    map<int, int>& inmp) {
+    if (preStart > preEnd || inStart > inEnd)
+        return NULL;
+    node* root = new node(preorder[preStart]);
+    int inroot = inmp[root->data];
+    int numsleft = inroot - inStart;
+    root->left = buildtree2pre(preorder, preStart + 1, preStart + numsleft,
+                               inorder, inStart, inroot - 1, inmp);
+    root->right = buildtree2pre(preorder, preStart + numsleft + 1, preEnd,
+                                inorder, inroot + 1, inEnd, inmp);
+    return root;
+}
+node* buildtreepre(vector<int>& inorder, vector<int>& preorder) {
+    map<int, int> inmp;
+    for (int i = 0; i < inorder.size(); i++) {
+        inmp[inorder[i]] = i;
+    }
+    node* root = buildtree2pre(preorder, 0, preorder.size() - 1, inorder, 0,
+                               inorder.size() - 1, inmp);
+    return root;
+}
+
+//when postorder and inorder vectors are given
+node* buildtree2post(vector<int>& postorder, int postStart, int postEnd,
+                     vector<int>& inorder, int inStart, int inEnd,
+                     map<int, int>& inmp) {
+    if (postStart > postEnd || inStart > inEnd)
+        return NULL;
+    node* root = new node(postorder[postEnd]);
+    int inroot = inmp[root->data];
+    int numsleft = inroot - inStart;
+    root->left = buildtree2post(postorder, postStart, postStart + numsleft - 1,
+                                inorder, inStart, inroot - 1, inmp);
+    root->right = buildtree2post(postorder, postStart + numsleft, postEnd - 1,
+                                 inorder, inroot + 1, inEnd, inmp);
+    return root;
+}
+node* buildtreepost(vector<int>& inorder, vector<int>& postorder) {
+    map<int, int> inmp;
+    for (int i = 0; i < inorder.size(); i++) {
+        inmp[inorder[i]] = i;
+    }
+    node* root = buildtree2post(postorder, 0, postorder.size() - 1, inorder, 0,
+                                inorder.size() - 1, inmp);
+    return root;
+}
+
+int main(){
+    vector<int> inorder_seq = {1, 2, 3, 4, 5};
+    vector<int> preorder_seq = {3, 2, 1, 4, 5};
+    vector<int> postorder_seq = {1, 2, 5, 4, 3};
+
+    cout << "=== Build BST from Preorder & Inorder ===\n";
+    node* tree_pre = buildtreepre(inorder_seq, preorder_seq);
+    vector<int> out;
+    inorder(tree_pre, out);
+    cout << "Inorder: ";
+    for (auto x : out) cout << x << " ";
+    out.clear();
+    cout << endl;
+    cout << "=== Build BST from Postorder & Inorder ===\n";
+    node* tree_post = buildtreepost(inorder_seq, postorder_seq);
+    inorder(tree_post, out);
+    cout << "Inorder: ";
+    for (auto x : out) cout << x << " ";
+}