114. Flatten Binary Tree to Linked List

一次AC，前序遍历再重构

/** * 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 preOrder(TreeNode* root,queue<TreeNode*>& nodes)    {        if(root==NULL)            return;        else        {            nodes.push(root);            if(root->left!=NULL)                preOrder(root->left,nodes);            if(root->right!=NULL)                preOrder(root->right,nodes);            return;        }    }    void flatten(TreeNode* root) {        if(root==NULL)            return;        queue<TreeNode*> nodes;        preOrder(root,nodes);        TreeNode* preRoot=nodes.front();        nodes.pop();        while(!nodes.empty())        {            preRoot->left=NULL;            preRoot->right=nodes.front();            preRoot=nodes.front();            nodes.pop();        }        preRoot->left=NULL;        preRoot->right=NULL;        return;    }};