[geeks]Construct Full Binary Tree from given preorder and postorder traversals

思路：根據先序和後序遍歷數據，遞歸的構造整棵樹

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> &postorder) {
		buildPos(postorder);
		return buildTree(preorder, 0, preorder.size()-1, postorder, 0, postorder.size()-1);
	}

	TreeNode *buildTree(vector<int> &pre, int l, int r, vector<int> &post, int b, int e) {
		if(l > r || b > e) return NULL;
		TreeNode *root = new TreeNode(pre[l]);
		if(l < r)
		{
			int postIdx = pos[pre[l+1]], preIdex = postIdx + l + 1;
			root->left = buildTree(pre, l+1, preIdex, post, b, postIdx);
			root->right = buildTree(pre, preIdex+1, r, post, postIdx+1, e-1);
		}
		return root;
	}
	void buildPos(vector<int> &postorder) {
		for(int i = 0; i < postorder.size(); ++i)
			pos[postorder[i]] = i;
	}
private:
	unordered_map<int, int> pos;
};