【Leetcode】【Medium】Construct Binary Tree from Preorder and Inorder Traversal
2015-05-21 05:33
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Given preorder and inorder traversal of a tree, construct the binary tree.
Note:
You may assume that duplicates do not exist in the tree.
解题思路:
给出一个二叉树的先序和中序遍历结果,还原这个二叉树。
对于一个二叉树:
先序遍历结果为:1 2 4 5 3 6 7
中序遍历结果为:4 2 5 1 6 3 7
由此可以发现规律:
1、先序遍历的第一个字符,就是根结点(1)
2、发现根节点后,对应在中序遍历中的位置,则在中序遍历队列中,根节点左边的元素构成根的左子树,根的右边元素构成根的右子树;
3、递归的将左右子树也按照上述规律进行构造,最终还原二叉树。
代码:
Note:
You may assume that duplicates do not exist in the tree.
解题思路:
给出一个二叉树的先序和中序遍历结果,还原这个二叉树。
对于一个二叉树:
1 / \ 2 3 / \ / \ 4 5 6 7
先序遍历结果为:1 2 4 5 3 6 7
中序遍历结果为:4 2 5 1 6 3 7
由此可以发现规律:
1、先序遍历的第一个字符,就是根结点(1)
2、发现根节点后,对应在中序遍历中的位置,则在中序遍历队列中,根节点左边的元素构成根的左子树,根的右边元素构成根的右子树;
3、递归的将左右子树也按照上述规律进行构造,最终还原二叉树。
代码:
/** * 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* buildTree(vector<int>& preorder, vector<int>& inorder) { return buildSubtree(preorder, inorder, 0, preorder.size()-1, 0, inorder.size()-1); } TreeNode* buildSubtree(vector<int>& preorder, vector<int>& inorder, int p_left, int p_right, int i_left, int i_right) { if (p_left > p_right || i_left > i_right) return NULL; int root = preorder[p_left]; TreeNode* node = new TreeNode(preorder[p_left]); int range = 0; for (int j = i_left; j <= i_right; ++j) { if (root == inorder[j]) { range = j - i_left; break; } } node->left = buildSubtree(preorder, inorder, p_left + 1, p_left + range, i_left, i_left + range - 1); node->right = buildSubtree(preorder, inorder, p_left + range + 1, p_right, i_left + range + 1, i_right); return node; } };
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