【算法刷题指南】队列+宽搜（BFS）

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val) {
        val = _val;
    }

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
public:
    vector<vector<int>> levelOrder(Node* root) {
        vector<vector<int>> ans;
        queue<Node*> q;
        q.push(root);
        if(root==nullptr) return ans;
        while(q.size())
        {
            int sz=q.size();
            vector<int> tmp;
            while(sz--)
            {
                Node* t=q.front();
                q.pop();
                tmp.push_back(t->val);
                for(Node* child:t->children)
                {
                    if(child!=nullptr) q.push(child);
                }
            }
            ans.push_back(tmp);
        }
        return ans;
    }
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> zigzagLevelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if(root==nullptr) return ans;

        queue<TreeNode*> q;
        q.push(root);
        int lev=1;

        while(q.size())
        {
            int sz=q.size();
            vector<int> tmp;
            while(sz--)
            {
                TreeNode* t=q.front();
                q.pop();
                tmp.push_back(t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);
            }
            if(lev%2==0) reverse(tmp.begin(),tmp.end());
            ans.push_back(tmp);
            lev++;
        }
        return ans;
    }
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
 * right(right) {}
 * };
 */
class Solution {
public:
    int widthOfBinaryTree(TreeNode* root) {
        vector<pair<TreeNode*, unsigned int>> q;
        q.push_back({root, 1});
        unsigned int ans = 0;
        while (q.size()) {
            auto& [x1, y1] = q[0];
            auto& [x2, y2] = q.back();
            ans = max(ans, y2 - y1 + 1);

            vector<pair<TreeNode*, unsigned int>> tmp;
            for (auto& [x, y] : q) {
                if (x->left)
                    tmp.push_back({x->left, y * 2});
                if (x->right)
                    tmp.push_back({x->right, y * 2 + 1});
            }
            q = tmp;
        }
        return ans;
    }
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> largestValues(TreeNode* root) {
        vector<int> ans;
        if(root==nullptr) return ans;
        queue<TreeNode*> q;
        q.push(root);

        while(q.size())
        {
            int sz=q.size();
            int _max=INT_MIN;
            while(sz--)
            {
                auto t=q.front();
                q.pop();

                _max=max(_max,t->val);
                if(t->left) q.push(t->left);
                if(t->right) q.push(t->right);

            }
            ans.push_back(_max);
        }
        return ans;
    }
};