LeetCode: Construct Binary Tree from Preorder and Inorder Traversal

思路：记录好中序、后序在数组中的其实位置和当前元素个数，计算好左右子树根节点在数组中的位置和左右子树元素个数，只要左右子树的个数都为0，就会递归终止，返回叶子节点的指针，重复这一过程就是构造出了树结构。

code:

class Solution {
public:
    TreeNode * buildTreePreInOrder(vector<int>::iterator preIt,vector<int>::iterator inIt, int num){
        TreeNode * root = new TreeNode(*preIt);
        vector<int>::iterator it = inIt;
        while(*it != *preIt)
            it++;
        int numLeft = it - inIt, numRight = num - numLeft - 1;
        if(numLeft > 0)
            root->left = buildTreePreInOrder(preIt+1,inIt,numLeft);
        if(numRight >0) 
            root->right = buildTreePreInOrder(preIt+numLeft+1,inIt+numLeft+1,numRight);
        return root;
    }
    TreeNode *buildTree(vector<int> &preorder, vector<int> &inorder) {
        TreeNode * root;
        if(!preorder.empty())
            root = buildTreePreInOrder(preorder.begin(),inorder.begin(),inorder.size());
        else
            return NULL;
        return root;
    }
};