leetcode-构造二叉树

</pre><p>1.由前序遍历和中序遍历构造二叉树</p><p><pre name="code" class="cpp">/**
 * 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 build(preorder,inorder,0,0,inorder.size()-1);
        
        
    }
    TreeNode* build(vector<int>& preorder, vector<int>& inorder,int pstart,int start,int end)
    {
        if(start>end) return NULL;
        TreeNode* root=new TreeNode(preorder[pstart]);
        int k;
        for(int i=start;i<=end;i++ )
        {
            if(inorder[i]==root->val){
                k=i;
                break;
            }
        }
        root->left=build(preorder,inorder,pstart+1,start,k-1);
        root->right=build(preorder,inorder,pstart+k+1-start,k+1,end);
        return root;
    }
};

2.由中序遍历和后序遍历构造二叉树

/**
 * 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>& inorder, vector<int>& postorder) {
        return build(inorder,postorder,postorder.size()-1,0,inorder.size()-1);
    }
    TreeNode* build(vector<int>& inorder, vector<int>& postorder,int pstart,int start,int end)
    {
        if(start>end) return NULL;
        TreeNode* root= new TreeNode(postorder[pstart]);
        int k;
        for(int i=start;i<=end;i++)
        {
            if(inorder[i]==root->val)
            {
                k=i;
                break;
            }
        }
        root->left=build(inorder,postorder,pstart+k-end-1,start,k-1);//这个地方的pstart很难确定
        root->right=build(inorder,postorder,pstart-1,k+1,end);
        return root;
    }
};