LeetCode热题100刷题9：25. K 个一组翻转链表、101. 对称二叉树、543. 二叉树的直径、102. 二叉树的层序遍历

25. K 个一组翻转链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* reverse(ListNode* a, ListNode* b) {
        ListNode* pre=nullptr;
        ListNode* cur = a;
        ListNode* nxt = a;
        while(cur!=b) {
            nxt = cur->next;
            cur->next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }
    ListNode* reverseKGroup(ListNode* head, int k) {
        if(head==nullptr)
            return nullptr;
        ListNode* a = head;
        ListNode* b = head;
        for(int i=0;i<k;i++) {
            if(b==nullptr)
                return head;
            b=b->next;
        }
        ListNode* newHead = reverse(a,b);
        a->next = reverseKGroup(b,k);
        return newHead;
    }
};

101. 对称二叉树

/**
 * 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:
    bool compare(TreeNode* left, TreeNode* right) {
        if(!right && !left)
            return true;
        if(left && !right)
            return false;
        if(right && !left)
            return false;
        if(left->val != right->val)
            return false;
        bool l = compare(left->left,right->right);
        bool r = compare(left->right,right->left);
        return l&&r;
    }
    bool isSymmetric(TreeNode* root) {
        if(!root)
            return true;
        return compare(root->left,root->right);
    }
};

543. 二叉树的直径

/**
 * 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 ans;
    int depth(TreeNode* root) {
        if(!root)
            return 0;
        int left = depth(root->left);
        int right = depth(root->right);
        ans = max(ans,left+right);
        return max(left,right)+1;
    }
    int diameterOfBinaryTree(TreeNode* root) {
        ans=0;
        depth(root);
        return ans;
    }
};

102. 二叉树的层序遍历

/**
 * 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>> levelOrder(TreeNode* root) {
        queue<TreeNode*> que;
        vector<vector<int>> res;
        if(root==NULL)
            return res;
        que.push(root);
        while(!que.empty()) {
            int size = que.size();
            vector<int> vec;
            for(int i=0;i<size;i++) {
                TreeNode* node = que.front();
                que.pop();
                vec.push_back(node->val);
                if(node->left!=NULL)
                    que.push(node->left);
                if(node->right!=NULL)
                    que.push(node->right);
            }
            res.push_back(vec);
        }
        return res;
    }
};