LeetCode·114. Flatten Binary Tree to Linked List

思路：先序遍歷

/**
 * 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 flatten(TreeNode* root) {
        if(root == NULL)return;
        stack<TreeNode*> st;
        st.push(root);
        TreeNode* fn = root;
        TreeNode* fr = root;
        while(!st.empty()){
            fn = st.top();
            if(fn != root){
                fr->right = fn;
                fr->left = NULL;
                fr = fn;
            }
            st.pop();
            if(fn->right != NULL)
                st.push(fn->right);
            if(fn->left != NULL)
                st.push(fn->left);
        }
    }
};