C++ 代码

/**
 * 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:
    void dfs(TreeNode * root)
    {
        if(!root) return;
        dfs(root->left);
        dfs(root->right);
        if(root->left)
        {
            TreeNode * tmp=root->right;
            root->right=root->left;
            root->left=NULL;
            TreeNode *right=root->right;
            TreeNode *pre=root->right;
            while(right)
            {
                pre=right;
                right=right->right;
            }
            pre->right=tmp;
        }
    }
    void flatten(TreeNode* root) {
        dfs(root);
    }
};