/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */

struct TreeNode* prev_node = NULL;

//先序遍历二叉树的每一个结点
//prev_node记录前一个遍历到的结点
void Pre_Order(struct TreeNode * T){
    if(T!=NULL){
        struct TreeNode* Left = T->left;
        struct TreeNode* Right = T->right;
        
        prev_node->left = NULL;
        prev_node->right = T;
        prev_node = T;
        
        Pre_Order(Left);//遍历左子树
        Pre_Order(Right);//遍历右子树

        //注：写成Pre_Order(T->Left); Pre_Order(T->Right);会出错
        //因为之前prev_node的操作会改变T->Right的值

    }
}

void flatten(struct TreeNode* root){
    prev_node = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    prev_node->val = 1000;
    Pre_Order(root);
}

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