题解 | 求二叉树的前序遍历

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 * };
 */

#include <stack>
#include <vector>
class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector
     */
    vector<int> preorderTraversal(TreeNode* root) {
        // write code here
	  	//利用栈的先进后出，先进右子树再进左子树，就能按顺序弹出左子树、右子树
        vector<int> result;
        if(root == nullptr) return result;

        stack<TreeNode*> st;
        st.push(root);
        while(!st.empty()){
            TreeNode *out = st.top();
            st.pop();
            result.push_back(out->val);

            if(out->right != nullptr){
                st.push(out->right);
            }
            if(out->left != nullptr){
                st.push(out->left);
            }
        }

        return result;
    }
};