LeetCode OJ 114. Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example,
Given

         1
        / \
       2   5
      / \   \
     3   4   6

The flattened tree should look like:

   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6

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解答

先序遍历同时把节点都堆到左边，因为先序先处理左子树，所以这样操作只是更改叶节点的left而不会对遍历有影响，最后把所有left赋值给right就可以了。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
 struct TreeNode *pre_node;
 void DFS(struct TreeNode *root){
     if(NULL == root){
        return;
    }
    if(pre_node != root){
        pre_node->left = root;
        pre_node = root;
    }
    DFS(root->left);
    DFS(root->right);
 }
void flatten(struct TreeNode* root) {
    pre_node = root;
    DFS(root);
    while(root != NULL){
        root->right = root->left;
        root->left = NULL;
        root = root->right;
    }
}