783. Minimum Distance Between BST Nodes

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
void solve(vector<int> &ans,TreeNode* root){
    if(root==NULL){
        return;
    }
    solve(ans,root->left);
    ans.push_back(root->val);
    solve(ans,root->right);
}
    int minDiffInBST(TreeNode* root) {
        if(!root){
            return NULL;
        }
        vector<int> ans;
        solve(ans,root);
        int mini=INT_MAX;
        for(int i=0;i<ans.size()-1;i++){
            mini=min(mini,ans[i+1]-ans[i]);
        }
        return mini;
    }
};