一些笔试常考简单算法小结

1.链表逆转

ListNode* ReverseList(ListNode* pHead){    //什么也不用检查    ListNode *pre = nullptr, *next = nullptr;    while (pHead)    {        //获取后一个        next = pHead->next;        //指针反转        pHead->next = pre;        //pre和pHead向后滑动        pre = pHead;        pHead = next;    }    return pre;}

2.二叉树遍历
前序

void PreOrderVistTree(TreeNode *root){    if (root == nullptr)return;    cout << root->val << endl;    PreOrderVistTree(root->left);    PreOrderVistTree(root->right);}

中序

void InOrderVistTree(TreeNode *root){    if (root == nullptr)return;    InOrderVistTree(root->left);    cout << root->val << endl;    InOrderVistTree(root->right);}

后序

void PostOrderVistTree(TreeNode *root){    if (root == nullptr)return;    PostOrderVistTree(root->left);    PostOrderVistTree(root->right);    cout << root->val << endl;}

深度优先遍历

void DepthVistFirst(TreeNode* root){    stack<TreeNode*>s;    s.push(root);    while (!s.empty())    {        root = s.top();        cout << root->val << endl;        s.pop();        if (root->right != nullptr)s.push(root->right);        if (root->left != nullptr)s.push(root->left);    }}

广度优先遍历

void BroadVistFirst(TreeNode* root){    queue<TreeNode*>q;    q.push(root);    while (!q.empty())    {        root = q.front();        cout << root->val << endl;        q.pop();        if (root->left != nullptr)q.push(root->left);        if (root->right != nullptr)q.push(root->right);    }}

二叉树的深度

int TreeDepth(BinaryTreeNode* pRoot)//计算二叉树深度{    if(pRoot==NULL)//如果pRoot为NULL，则深度为0，这也是递归的返回条件        return 0;    //如果pRoot不为NULL，那么深度至少为1，所以left和right=1    int left=1;    int right=1;    left+=TreeDepth(pRoot->left);//求出左子树的深度    right+=TreeDepth(pRoot->right);//求出右子树深度    return left>right?left:right;//返回深度较大的那一个}//简化版int TreeDepth(TreeNode* pRoot){   if(!pRoot) return 0 ;   return max(1+TreeDepth(pRoot->left), 1+TreeDepth(pRoot->right));}

判断是否平衡二叉树

bool IsBalanced(TreeNode* pRoot){    if (pRoot == NULL)return true;    //分别求左右子树深度    int leftDepth = TreeDepth(pRoot->left);    int rightDepth = TreeDepth(pRoot->right);    int diff = rightDepth - leftDepth;    //深度比较    if (diff>1 || diff<-1)        return false;    //递归比较判别    return IsBalanced(pRoot->left) && IsBalanced(pRoot->right);}

其中，树的结构如下：

struct TreeNode{    int val;    struct TreeNode *left;    struct TreeNode *right;    TreeNode(int x) :val(x), left(nullptr), right(nullptr) {}};