144. 二叉树的前序遍历

方法一：递归

/**
 * 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 {
private:
    vector<int> res;     //存储遍历数据
public:
    vector<int> preorderTraversal(TreeNode* root) {   
        if(root){
            res.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return res;
    }
};

/**
 * 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;   
        preTravel(root, res);   //传递res参数保存返回值
        return res;
    }
private:
    void  preTravel(TreeNode* root,vector<int> &vc){
        if(!root)
            return;
        vc.push_back(root->val);
        preTravel(root->left,vc);
        preTravel(root->right,vc);
    }
};

方法二：迭代，栈

/**
 * 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;       
        st.push(root);
        while(!st.empty()){
            TreeNode *top=st.top();
            st.pop();
            if(top){
                res.push_back(top->val);
                st.push(top->right);	//注意右子树先入栈，左子树后入栈
                st.push(top->left);
            }
        }        
        return res;
    }
};