前序遍歷：根左右

//用棧來實現非遞歸解法
/**
 * 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> preorderTraversal(TreeNode* root) {
        vector
 
<int> res;
        if(root == NULL)
            return res;
        stack<TreeNode*> stack;
        stack.push(root);
        
        while(!stack.empty()){
            TreeNode* c = stack.top();
            stack.pop();
            res.push_back(c->val);
            if(c->right)
                stack.push(c
 
->right);
            if(c->left)
                stack.push(c->left);
        }
        return res;
    }
};

//遞歸解法，註意res是全局的，要放在遍歷函數外面
/**
 * 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) {
        
        if(root){
            res.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        return res;
    }
};

中序遍歷：左根右

/**
 * 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> inorderTraversal(TreeNode* root) {
        stack<TreeNode*> st;
        vector<int> res;
        if(root == NULL) return res;  //若根節點為空則返回空
        TreeNode* p = root;
        while(p || !st.empty()){
            while(p){
                //先將根節點入棧，再將它所有的左節點入棧
                st.push(p);
                p = p->left;   
            }
            
            p = st.top();
            st.pop();
            res.push_back(p->val);
            p = p->right;
        }
        return res;
    }
};

棧和遞歸的關系 144:Binary Tree Preorder Traversal