144-二叉树的前序遍历

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

class Solution {

void preorderTraversal(TreeNode *root, vector<int> &v) {

if (root == nullptr) {

return;

}

v.push_back(root->val);

if (root->left) {

preorderTraversal(root->left, v);

}

if (root->right) {

preorderTraversal(root->right, v);

}

public:

vector<int> preorderTraversal(TreeNode* root) {

vector<int> v;

preorderTraversal(root, v);

return v;

}

};

144-二叉树的前序遍历-图片1

3.2 非递归代码

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode *> st;
        TreeNode *p;
        vector<int> v;

        p = root;
        while (p || !st.empty()) {
            while (p) {
                // 遍历当前节点
                v.push_back(p->val);
                st.push(p);
                // 遍历左子树
                p = p->left;
            }

            // 通过栈来遍历右子树
            if (!st.empty()) {
                p = st.top()->right;
                st.pop();
            }
        }

        return v;
    }
};