105.Construct Binary Tree from Preorder and Inorder Traversal

思路：

• 递归。

/**
 * 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 Construct(preorder,0,preorder.size()-1,inorder,0,inorder.size()-1);
    }
    TreeNode *Construct(vector<int>& preorder,int s0,int e0,vector<int>& inorder,int s1,int e1){
        if(s0 > e0 || s1 > e1)  return NULL;
        int rootval = preorder[s0];
        int rootindex = find(inorder,s1,e1,rootval);
        TreeNode* root = new TreeNode(rootval);
        int leftsize = rootindex - s1 + 1;
        
        root->left = Construct(preorder,s0+1,s0+leftsize-1,inorder,s1,rootindex-1);
        root->right = Construct(preorder,s0+leftsize,e0,inorder,rootindex+1,e1);
        return root;
    }
    int find(vector<int>& inorder,int s1,int e1,int rootval){
        for(int i = s1; i <= e1; i++){
            if(inorder[i] == rootval) return i;
        }
        return -1;
    }
};