二叉树遍历（前中后）

二叉树前序遍历：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> res;
        if(root==NULL)return res;
        stack<TreeNode *> sta;
        sta.push(root);
        while(!sta.empty())
        {
            TreeNode *tmp=sta.top();
            sta.pop();
            res.push_back(tmp->val);
            if(tmp->right)sta.push(tmp->right);
            if(tmp->left)sta.push(tmp->left);
        }
        return res;
    }
/**
void preorder(vector<int> &res, TreeNode * root)
{
    if(root==NULL)return;
    res.push_back(root->val);
    preorder(res,root->left);
    preorder(res,root->right);
}
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> res;
        preorder(res,root);
        return res;
    }
*/
};

二叉树中序遍历：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
vector<int> inorderTraversal(TreeNode *root)
{
    vector<int> res;
    if(root==NULL)return res;
    TreeNode *p=root;
    stack<TreeNode *> sta;
    while(p!=NULL||!sta.empty())
    {
        while(p!=NULL)
        {
            sta.push(p);
            p=p->left;
        }
        if(!sta.empty())
        {
            p=sta.top();
            sta.pop();
            res.push_back(p->val);
            p=p->right;
        }
    }
    return res;
}
/*
void inorder(TreeNode *root, vector<int> &res)
{
    if(root==NULL)return;
    inorder(root->left,res);
    res.push_back(root->val);
    inorder(root->right,res);
    return;
}
    vector<int> inorderTraversal(TreeNode *root) {
        vector<int> res;
        inorder(root,res);
        return res;
    }
*/
};

二叉树后序遍历：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
        if(root==NULL)return res;
        map<TreeNode *, int> smap;
        smap[root]=0;
        stack<TreeNode *> sta;
        sta.push(root);
        while(!sta.empty())
        {
            TreeNode *p=sta.top();
            if((!p->right&&!p->left)||smap.count(p->right)||smap.count(p->left))
            {
                res.push_back(p->val);
                smap[p]=1;
                sta.pop();
            }
            else
            {
               if(p->right&&!smap.count(p->right))
               {
                sta.push(p->right);
                smap[p->right]=0;
               }
               if(p->left&&!smap.count(p->left))
               {
                sta.push(p->left);
                smap[p->left]=0;
               }
            }
        }
        return res;
    }
/**
void postorder(vector<int> & res,TreeNode *root)
{
    if(root==NULL)return;
    postorder(res,root->left);
    postorder(res,root->right);
    res.push_back(root->val);
}
    vector<int> postorderTraversal(TreeNode *root) {
        vector<int> res;
        postorder(res,root);
        return res;
    }
*/
};