建议先阅读二叉树的中序遍历

前序遍历顺序为根 -> 左 -> 右，所以运用递归算法时，先将该点放入遍历数组，再递归遍历左子树和右子树

递归算法

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> res;
    vector<int> preorderTraversal(TreeNode* root) {
        dfs(root);
        return res;
    }

    void dfs(TreeNode* p)
    {
        if (!p) return;
        res.push_back(p -> val);
        dfs(p -> left);
        dfs(p -> right);
    }
};

迭代算法

当前遍历结点即为根，所以在迭代算法中，先将该点放入遍历数组，然后将左子树链递到最低层，然后出栈，放入右子树。

/**
 * Definition for a binary tree node.
 * 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;
        stack<TreeNode*> st;
        auto p = root;

        while (p || !st.empty())
        {
            while (p)
            {
                res.push_back(p -> val);
                st.push(p);
                p = p -> left;
            }

            p = st.top();
            st.pop();
            p = p-> right;
        }

        return res;
    }
};