144:Binary Tree Preorder Traversal【樹】【棧】

題目鏈接：click~

遞歸解法：

/*題意：二叉樹的前序遍歷 */
class Solution {
public:
    void PreOrderTraverse(TreeNode *T, vector<int> &v) {
        if(T == NULL) return;
        v.push_back(T->val);
        PreOrderTraverse(T->left, v);
        PreOrderTraverse(T->right, v);
    }
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> v;
        PreOrderTraverse(root, v);
        return v;
    }
};

棧解法：

class Solution {
private:
    vector<int> v;
public:
    void pushleft(TreeNode *root, stack<TreeNode*> &s) {
        if(root != NULL) {
            s.push(root);
            v.push_back(root->val);
            pushleft(root->left, s);
        }
    }
    vector<int> preorderTraversal(TreeNode *root) {
        if(root == NULL) return v;
        stack<TreeNode*> s;
        pushleft(root, s);
        while(!s.empty()) {
            TreeNode *tmp = s.top();
            s.pop();
            pushleft(tmp->right, s);
        }
        return v;
    }
};