模板：

1.先序遍歷三種方法

1）叠代：

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */    
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> res;
        stack<TreeNode*> stack;
        
        
 
if (root == NULL) {
            return res;
        }
        
        stack.push(root);
        while (!stack.empty()) {
            TreeNode *node = stack.top();
            stack.pop();
            res.push_back(node->val);
            if (node->right != NULL) {
                stack.push(node
 
->right);
            }
            if (node->left != NULL) {
                stack.push(node->left);
            }
        }
        
        return res;
    }
};

2）遞歸：

class Solution {
public:
    void traversal(TreeNode *root, vector<int>& res) {
        if (root == NULL) {
            
 
return;
        }
        
        res.push_back(root->val);
        traversal(root->left, res);
        traversal(root->right, res);
    }
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> res;
        traversal(root, res);
        
        return res;
    }
};

3）分治：

class Solution {
public:    
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> res;
        if (root == NULL) {
            return res;
        }
        
        vector<int> left = preorderTraversal(root->left);
        vector<int> right = preorderTraversal(root->right);
        
        res.push_back(root->val);
        res.insert(res.end(), left.begin(), left.end());
        res.insert(res.end(), right.begin(), right.end());
        
        return res;
    }
};

【算法模板】二叉樹