这类题属于面试常考题，递归法实现容易，尤其要注意迭代法。

前序遍历

递归法

class Solution {
public:
    void preorder(TreeNode* root, vector<int>& res)
    {
        if (!root) return;

        res.push_back(root->val);
        preorder(root->left, res);
        preorder(root->right, res);
    }

    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        preorder(root, res);
        return res;
    }
};

迭代法

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*> stk;
        vector<int> res;

        if (!root) return res;
        stk.push(root);

        while (stk.size())
        {
            auto t = stk.top();
            stk.pop();
            res.push_back(t->val);

            if (t->right) stk.push(t->right);   //栈是先进后出，所以右节点先进
            if (t->left) stk.push(t->left);
        }

        return res;
    }
};

中序遍历

递归法

class Solution {
public:
    void inorder(TreeNode* root, vector<int>& res)
    {
        if (!root) return;

        inorder(root->left, res);
        res.push_back(root->val);
        inorder(root->right, res);
    }

    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        inorder(root, res);
        return res;
    }
};

迭代法

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> stk;
        auto p = root;

        while (p || stk.size())     //p指针不为空或栈不为空
        {
            if (p)                  //往左子树走
            {
                stk.push(p);
                p = p->left;
            }
            else                    //左子树没有了，回退同时往右子树走
            {
                p = stk.top();
                stk.pop();
                res.push_back(p->val);
                p = p->right;
            }
        }

        return res;
    }
};

后序遍历

递归法

class Solution {
public:
    void dfs(TreeNode* root, vector<int>& res)
    {
        if (!root) return;

        dfs(root->left, res);
        dfs(root->right, res);
        res.push_back(root->val);
    }

    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        dfs(root, res);
        return res;
    }
};

迭代法

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        stack<TreeNode*> stk;
        vector<int> res;

        if (!root) return res;
        stk.push(root);

        while (stk.size())
        {
            auto t = stk.top();
            stk.pop();
            res.push_back(t->val);
            if (t->left) stk.push(t->left);
            if (t->right) stk.push(t->right);
        }

        reverse(res.begin(), res.end());        //最后要翻转
        return res;
    }
};