Add Solution for 109 Convert Sorted List to Binary Search Tree

cpp/109_Convert_Sorted_List_to_Binary_Search_Tree.cpp

@@ -0,0 +1,190 @@
+// 109. Convert Sorted List to Binary Search Tree
+/**
+ * Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
+ * 
+ * Tags: Depth-first Search, Linked List
+ * 
+ * Similar Problems: (E) Convert Sorted Array to Binary Search Tree
+ * 
+ * Author: Kuang Qin
+ */
+
+#include <iostream>
+#include <vector>
+#include <string>
+#include <sstream>
+#include <queue>
+
+using namespace std;
+
+/**
+ * Definition for singly-linked list.
+ */
+struct ListNode {
+    int val;
+    ListNode *next;
+    ListNode(int x) : val(x), next(NULL) {}
+    ListNode(int x, ListNode *p) : val(x), next(p) {}
+};
+
+/**
+ * Definition for a binary tree node.
+ */
+struct TreeNode {
+    int val;
+    TreeNode *left;
+    TreeNode *right;
+    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+};
+
+class Solution {
+public:
+    TreeNode* sortedListToBST(ListNode* head) {
+        if (head == NULL) {
+            return NULL;
+        }
+
+        if (head->next == NULL) {
+            return new TreeNode(head->val);
+        }
+
+        // find the middle point
+        ListNode *slow = head, *fast = head, *pre = head;
+        while (fast != NULL && fast->next != NULL) {
+            pre = slow;
+            slow = slow->next;
+            fast = fast->next->next;
+        }
+        pre->next = NULL;   // break the list into two halves
+
+        // use slow as root and build left and right sub-tree
+        TreeNode *root = new TreeNode(slow->val);
+        root->left = sortedListToBST(head);
+        root->right = sortedListToBST(slow->next);
+
+        return root;
+    }
+};
+
+class TreeOperation {
+    int getTreeHeight(TreeNode* root) {
+        if (root == NULL) {
+            return 0;
+        }
+
+        int l = getTreeHeight(root->left);
+        int r = getTreeHeight(root->right);
+
+        if (l > r) {
+            return l + 1;
+        }
+
+        return r + 1;
+    }
+
+    // output all the node in level order, including null pointers
+    vector<vector<string>> levelOrderFull(TreeNode* root) {
+        vector<vector<string>> output;
+        if (root == NULL) {
+            return output;
+        }
+
+        int h = getTreeHeight(root);
+        queue<TreeNode*> q;
+        q.push(root);
+
+        // fill the container in each level
+        for (int i = 0; i < h; i++) {
+            vector<string> level;
+            int currLevelCount = q.size();
+
+            // while loop for current level
+            while (currLevelCount--) {
+                TreeNode *curr = q.front();
+                if (curr == NULL) {
+                    level.push_back("&");
+                    q.push(NULL);
+                    q.push(NULL);
+                }
+                else {
+                    stringstream ss;
+                    ss << curr->val;
+                    level.push_back(ss.str());
+                    q.push(curr->left);
+                    q.push(curr->right);
+                }
+
+                q.pop();
+            }
+
+            output.push_back(level);
+        }
+
+        return output;
+    }
+public:
+    void printTree(TreeNode *root) {
+        vector<vector<string>> output = levelOrderFull(root);
+        int h = output.size();
+        if (h == 0) {
+            return;
+        }
+
+        int w = 2 * output[h - 1].size() + 1;   // total width
+        for (int i = 0; i < h; i++) {
+            int n = output[i].size();
+            int m = (w - n) / (n + 1);  // calculate space width
+            string sp(m, ' ');
+            if ((w - n) % (n + 1)) {
+                cout << sp << " ";
+            }
+            else {
+                cout << sp;     // add space
+            }
+
+            for (int j = 0; j < n; j++) {
+                cout << output[i][j] << sp;
+            }
+            cout << endl;
+        }
+
+        return;
+    }
+
+    void deleteTree(TreeNode *root) {
+        if (root == NULL) {
+            return;
+        }
+
+        if (root->left == NULL && root->right == NULL) {
+            delete root;
+            return;
+        }
+
+        deleteTree(root->left);
+        deleteTree(root->right);
+        return;
+    }
+};
+
+int main() {
+    ListNode node10(10), node9(9, &node10), node8(8, &node9), node7(7, &node8), node6(6, &node7);
+    ListNode node5(5, &node6), node4(4, &node5), node3(3, &node4), node2(2, &node3), node1(1, &node2);
+    ListNode *head = &node1, *curr = head;
+    cout << "Sorted List: " << endl;
+    while (curr != NULL) {
+        cout << curr->val << " ";
+        curr = curr->next;
+    }
+    cout << endl;
+
+    Solution sol;
+    TreeNode *root = sol.sortedListToBST(head);
+    cout << "Binary Search Tree: " << endl;
+    TreeOperation treeop;
+    treeop.printTree(root);
+    treeop.deleteTree(root);
+    cin.get();
+
+    return 0;
+}