Construct Binary Tree from Preorder and Inorder Traversal.

Author: AnnieKim

ConstructBinaryTreefromPreorderandInorderTraversal.h

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+/*
+ Author:     Annie Kim, [email protected]
+ Date:       May 16, 2013
+ Problem:    Construct Binary Tree from Preorder and Inorder Traversal
+ Difficulty: Easy
+ Source:     http://leetcode.com/onlinejudge#question_105
+ Notes:
+ Given preorder and inorder traversal of a tree, construct the binary tree.
+ Note:
+ You may assume that duplicates do not exist in the tree.
+
+ Solution: Recursion.
+ */
+
+/**
+ * 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 N = inorder.size();
+        return buildTreeRe(preorder, 0, N, inorder, 0, N);
+    }
+
+    TreeNode *buildTreeRe(vector<int> &preorder, int pre_s, int pre_e, 
+                          vector<int> &inorder, int in_s, int in_e) {
+        int N = in_e - in_s;
+        if (N <= 0) return NULL;
+        int pos = 0;
+        while (pos < N && inorder[in_s + pos] != preorder[pre_s]) pos++;
+
+        TreeNode *root = new TreeNode(preorder[pre_s]);
+        root->left = buildTreeRe(preorder, pre_s + 1, pre_s + 1 + pos, 
+                                 inorder, in_s, in_s + pos);
+        root->right = buildTreeRe(preorder, pre_s + 1 + pos, pre_e, 
+                                 inorder, in_s + pos + 1, in_e);
+        return root;
+    }
+};