输入:
​
   1
    \
     3
    /
   2
​
输出:
1

/**
 * 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:
    int getMinimumDifference(TreeNode* root) {
           int m=std::numeric_limits<int>::max();
        if(!root) return m;
        int ld=m,rd=m;
        if(root->left)
        {
            TreeNode* leftNode=root->left;
            while(leftNode)
            {
                ld=root->val - leftNode->val;
                leftNode=leftNode->right;
            }
​
​
        }
        if(root->right)
        {
            TreeNode* rightNode=root->right;
            while(rightNode)
            {
                rd=rightNode->val-root->val;
                rightNode=rightNode->left;
            }
​
​
        }
​
​
      //  cout<<ld << " " <<rd << " "<< m<<endl;
        if(root->left&& root->right) 
        {
            m=min(ld,rd);
            m=min(m,min(getMinimumDifference(root->left),getMinimumDifference(root->right)));
        }
        else if (root->left) 
            m=min(ld,getMinimumDifference(root->left));
        else if(root->right) m=min(rd,getMinimumDifference(root->right));
        else 
            return m;
      //  cout<<m<<endl;
        return m;
    }
};