0144-binary-tree-preorder-traversal.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 * int val;
 * TreeNode *left;
 * TreeNode *right;
 * TreeNode() : val(0), left(nullptr), right(nullptr) {}
 * TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
*/


classSolution {
public:
 vector<int> preorderTraversal(TreeNode* root) {
 stack<TreeNode *> stk;
 vector<int> ans;
for (auto cur = root; cur || !stk.empty();) {
for (; cur; cur = cur->left) {
 ans.emplace_back(cur->val);
 stk.emplace(cur);
 }
 cur = stk.top()->right;
 stk.pop();
 }
return ans;
 }
};