二叉树的基本遍历算法

基本数据结构

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

前序遍历

class Solution {public: vector<int> preorderTraversal(TreeNode *root)    {        vector<int> visit;        prevIterativeTraversal(root, visit);        return visit;    }        void prevTraversal(TreeNode * root, vector<int> &visit)    {        if(root == NULL)            return;        else        {            visit.push_back(root->val);             prevTraversal(root->left, visit);            prevTraversal(root->right,visit);        }    }    void prevIterativeTraversal(TreeNode *root, vector<int> &visit)    {        stack<TreeNode*> nodeStack;        TreeNode *iterator = root;        TreeNode *prev = root;        iterator = root;        while(iterator != NULL || !nodeStack.empty())        {            while(iterator != NULL)            //一直搜索该结点的左孩子            {                prev = iterator;                nodeStack.push(iterator);                visit.push_back(iterator->val);                iterator = iterator->left;            }            iterator = nodeStack.top();  //弹出结点            nodeStack.pop();             iterator = iterator->right;   //访问该结点的右孩子        }    }};

中序遍历

class Solution {public:    vector<int> midorderTraversal(TreeNode *root)    {        vector<int> visit;        midIterativeTraversal(root, visit);        return visit;    }    void midTraversal(TreeNode * root, vector<int> &visit)    {        if(root == NULL)            return;        else        {            midTraversal(root->right,visit);            visit.push_back(root->val);             midTraversal(root->left, visit);        }    }    void midIterativeTraversal(TreeNode *root, vector<int> &visit)    {        stack<TreeNode*> nodeStack;        TreeNode *iterator = root;        TreeNode *prev = root;        iterator = root;        while(iterator != NULL || !nodeStack.empty())          {            while(iterator != NULL)                 {                prev = iterator;                nodeStack.push(iterator);                iterator = iterator->left;            }            iterator = nodeStack.top();            visit.push_back(iterator->val);    //当该节点的右子树访问完毕的时候访问该节点            nodeStack.pop();            iterator = iterator->right;        }    }};

后序遍历

class Solution {public:    vector<int> postorderTraversal(TreeNode *root)    {        vector<int> visit;        postIterativeTraversal(root, visit);        return visit;    }    void postTraversal(TreeNode * root, vector<int> &visit)    {        if(root == NULL)            return;        else        {            postTraversal(root->left, visit);            postTraversal(root->right,visit);            visit.push_back(root->val);         }    }    void postIterativeTraversal(TreeNode *root, vector<int> &visit)    {        stack<TreeNode*> nodeStack;        TreeNode *iterator = root;        TreeNode *prev = root;        iterator = root;        while(iterator != NULL || !nodeStack.empty())        {            while(iterator != NULL)           //如果该结点不为空,那么一直寻找左孩子结点            {                prev = iterator;                nodeStack.push(iterator);                iterator = iterator->left;            }            prev = iterator;            iterator = nodeStack.top();                 while (iterator->right == prev)   //判断当前结点的右子树是否已经遍历完毕            {                nodeStack.pop();                            visit.push_back(iterator->val);     //当右子树遍历完毕便可以写入该结点的值                if(nodeStack.empty())                    return;                prev = iterator;                iterator = nodeStack.top();            }            prev = iterator;            iterator = iterator->right;        }    }};

层次遍历

class Solution {public:    vector<int> levelorderTraversal(TreeNode *root)    {        vector<int> visit;        queue<TreeNode*> nodeQueue;        TreeNode *iterator = NULL;        if(root != NULL)            nodeQueue.push(root);        while (!nodeQueue.empty())        {            iterator = nodeQueue.front();            nodeQueue.pop();            visit.push_back(iterator->val);            if (iterator->left != NULL)                nodeQueue.push(iterator->left);            if (iterator->right != NULL)                nodeQueue.push(iterator->right);        }        return visit;    }};