先序遍历和后序遍历构建二叉树

递归的方法利用先序遍历和中序遍历构建二叉树，同样也可以利用到中序遍历和后序遍历构建二叉树。

//利用先序遍历和中序遍历构建二叉树
TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
    TreeNode *root=NULL;
    if(preorder.size()==0||inorder.size()==0||preorder.size()!=inorder.size())
        return root;
    root=tree(preorder,0,preorder.size()-1,inorder,0,inorder.size()-1); 
    return root;
} 
TreeNode* tree(vector<int>&preorder,int preBegin,int preEnd,vector<int>&inorder,int inBegin,int inEnd){
    TreeNode *root=NULL;
    if(preorder.size()==0||inorder.size()==0||preorder.size()!=inorder.size())
        return root;
    TreeNode *leftNode=NULL;
    TreeNode *rightNode=NULL;
    root->val=preorder[preBegin];
    int rootVal=root->val;
    int i;
for(i=inBegin;i<=inEnd;i++)
    if(inorder[i]==root->val)
            break;
    if(i>inEnd)
        return root;
    int leftLen=i-inBegin;
    int rightLen=inEnd-i-1;
    leftNode=buildTree(preorder,preBegin+1,preBegin+leftLen,inorder,inBegin,inBegin+leftLen);
    rightNode=buildTree(preorder,preBegin+1+leftLen,preEnd,inorder,i+1,i+rightLen);
    root->left=leftNode;
    root->right=rightNode;
    return root;
}