preorder and inorder traversal

Author: zhongn2

Construct Binary Tree from Preorder and Inorder Traversal.cpp

@@ -0,0 +1,33 @@
+//Construct Binary Tree from Preorder and Inorder Traversal
+/**
+ * Definition for binary tree
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
+        int prelen = preorder.size();
+        int inlen = inorder.size() - 1;
+        if(prelen == 0 || inlen == 0) return NULL;
+        return newTree(preorder, inorder, 0, inlen, 0);
+    }
+    TreeNode *newTree(vector<int> &preorder, vector<int> &inorder, int inst, int inend, int prest){
+        if(inst > inend) return NULL;
+        TreeNode *node = new TreeNode(preorder[prest]);
+        int pos = -1;
+        for(int i = 0; i <= inorder.size(); i++){
+            if(inorder[i] == preorder[prest]){
+                pos = i;
+                break;
+            }
+        }
+        node -> left = newTree(preorder, inorder, inst, pos -1, prest + 1); // the second root is prest + 1
+        node -> right = newTree(preorder, inorder, pos + 1, inend, prest + 1 + pos - inst); // mid - inst is the size of left side
+        return node;
+    }
+};

Unique Binary Search Trees II.cpp

@@ -0,0 +1,41 @@
+//Unique Binary Search Trees II
+// recursion  generate the tree ( left, right) 
+// the same way as the last question
+
+//**
+ * Definition for binary tree
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    vector<TreeNode *> generateTrees(int n) {
+        if( n == 0) return generate(1, 0);
+        return generate(1, n);
+    }
+    
+    vector<TreeNode *>generate(int start, int end){
+        vector<TreeNode *> result;
+        if( start > end){
+            result.push_back(NULL);
+            return result;
+        }
+        for(int k = start; k <= end; k++){
+            vector<TreeNode *> l = generate(start, k- 1);
+            vector<TreeNode *> r = generate(k + 1, end);
+            for(auto i : l){
+                for(auto j : r){
+                     TreeNode *subnode = new TreeNode(k);
+                     subnode -> left = i;
+                     subnode -> right = j;
+                     result.push_back(subnode);
+                }
+            }
+        }
+        return result;
+    }
+};