STL

双端队列

Posted by Liao on 2022-06-17

双端队列

一. 定义

双端队列支持前后双端插入元素、弹出、删除元素

deque<object_type>deq

常用函数：

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push_back();//队尾添加元素

push_front(); //队列开头添加元素

TreeNode *node = deq.front();

pop_front(); // 删除队列中第一个元素

pop_back();//删除队列中第一个元素

back(); // 获取队列最后一个元素

front(); //获取队列第一个元素

二、例题

二叉树的镜像

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/**
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* mirrorTree(TreeNode* root) {
        deque<TreeNode*>deq;
        if(root == NULL) {
            return NULL;
        }
        deq.push_back(root);
        while(!deq.empty()) {
            TreeNode *node = deq.front();
            deq.pop_front(); 
            if(node->left) {
                deq.push_front(node->left);
            }
            if(node->right) {
                deq.push_front(node->right);
            }
            TreeNode *tmp = node->left;
            node->left = node->right;
            node->right = tmp;
        }

        return root;
    }
};

剑指 Offer 32 - III. 从上到下打印二叉树 III

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/**
 * 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:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>>ans;
        if(root == NULL) {
            return ans;
        }
        queue<TreeNode*>que;
        que.push(root);
        int level = 0;
        while(!que.empty()) {
            int size = que.size();
            deque<int>deq;
            for(int i = 0; i < size; i++) {
                TreeNode *node = que.front();
                que.pop();
                if(level % 2 == 0) { // 偶数    
                    deq.push_back(node->val);
                }
                else {
                    deq.push_front(node->val);
                }
                if(node->left) {
                    que.push(node->left);
                }
                if(node->right) {
                    que.push(node->right);
                }
            }
            ans.emplace_back(deq.begin(), deq.end());
            level++;
        }
        return ans;
    }
};
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