剑指Offer：从上往下打印二叉树

从上往下打印二叉树

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从上往下打印出二叉树的每个节点，同层节点从左至右打印。

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总觉得这题在LeetCode上面做过，待优化。

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/*struct TreeNode {    int val;    struct TreeNode *left;    struct TreeNode *right;    TreeNode(int x) :            val(x), left(NULL), right(NULL) {    }};*/class Solution {public:    vector<int> PrintFromTopToBottom(TreeNode *root) {        //BFS        stack<TreeNode*> st1,st2,st3;        vector<int> res{};        if(!root)   return res;        st1.push(root);        while((!st1.empty())||(!st2.empty())){            if(st1.empty()){                while(!(st2.empty())){                    TreeNode *cur=st2.top();                    st2.pop();                    res.push_back(cur->val);                    st3.push(cur);//st3辅助栈，逆序             //       if(cur->right) st1.push(cur->right);              //      if(cur->left)  st1.push(cur->left);                }                while(!(st3.empty())){                    TreeNode *cur=st3.top();                    st3.pop();                    if(cur->right)   st1.push(cur->right);                    if(cur->left)    st1.push(cur->left);                }            }            else{                while(!(st1.empty())){                    TreeNode *cur=st1.top();                    st1.pop();                    res.push_back(cur->val);                    if(cur->right)   st2.push(cur->right);                    if(cur->left)    st2.push(cur->left);                }            }        }        return res;    }};