73 前序遍历和中序遍历树构造二叉树

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param inorder: A list of integers that inorder traversal of a tree
     * @param postorder: A list of integers that postorder traversal of a tree
     * @return: Root of a tree
     */
    TreeNode * buildTree(vector<int> &preorder, vector<int> &inorder) {
        // write your code here
    int a=inorder.size();
    int b=preorder.size();
    if (a!=b||a==0||b==0)
    {
        return NULL;
    }

    return buildtr(preorder,inorder,0,b-1,0,a-1);
    }
    
    TreeNode *buildtr(vector<int> &preorder, vector<int> &inorder,int prst,int pred,int inst,int ined)
{
    if (inst>ined)
    {
        return NULL;
    }
    int i=inst;
    while(i<=ined&&inorder[i]!=preorder[prst])
    {
        i++;
    }
    TreeNode * root=new TreeNode(preorder[prst]);
    root->left=buildtr(preorder,inorder,prst+1,prst+i-inst,inst,i-1);//左子树，递归时弄清楚索引参数;
    root->right=buildtr(preorder,inorder,prst+i-inst+1,prst+ined-inst,i+1,ined);//右子树，递归时弄清楚索引参数;
    return root;
}
    
};