LeetCode 1008. 前序遍历构造二叉搜索树

根据左右划分子树进行建树；

class Solution {
public:
    TreeNode* bstFromPreorder(vector& preorder) {
        return buildTree(preorder, 0, preorder.size()-1);
    }

    TreeNode* buildTree(vector& preorder,int l,int r){
        if(l>r)
            return nullptr;
        TreeNode* ph=new TreeNode(preorder[l]);
        if(l==r)
            return ph;
        int end=r;
        l++;
        while(r>=l&&preorder[r]>ph->val){
            r--;
        }
        ph->left=buildTree(preorder,l,r);
        ph->right=buildTree(preorder, r+1, end);
        return ph;
    }
};