Traversal

Preorder

144. Binary Tree Preorder Traversal

See more my solutions here

// OJ: https://leetcode.com/problems/binary-tree-preorder-traversal/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        if (!root) return {};
        vector<int> ans;
        stack<TreeNode*> s{{root}};
        while (s.size()) {
            root = s.top();
            s.pop();
            ans.push_back(root->val);
            if (root->right) s.push(root->right);
            if (root->left) s.push(root->left);
        }
        return ans;
    }
};

Or

// OJ: https://leetcode.com/problems/binary-tree-preorder-traversal/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> ans;
        stack<TreeNode*> s;
        while (root || s.size()) {
            if (!root) {
                root = s.top();
                s.pop();
            }
            ans.push_back(root->val);
            if (root->right) s.push(root->right);
            root = root->left;
        }
        return ans;
    }
};

See also 589. N-ary Tree Preorder Traversal (Easy)

Inorder

94. Binary Tree Inorder Traversal (Easy)

See more my solutions here

// OJ: https://leetcode.com/problems/binary-tree-inorder-traversal/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> ans;
        stack<TreeNode*> s;
        while (root || s.size()) {
            while (root) {
                s.push(root);
                root = root->left;
            }
            root = s.top();
            s.pop();
            ans.push_back(root->val);
            root = root->right;
        }
        return ans;
    }
};

Postorder

145. Binary Tree Postorder Traversal (Easy)

See more my solutions here

// OJ: https://leetcode.com/problems/binary-tree-postorder-traversal/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> ans;
        stack<TreeNode*> s;
        TreeNode *prev = nullptr;
        while (root || s.size()) {
            while (root) {
                s.push(root);
                root = root->left;
            }
            root = s.top();
            if (!root->right || root->right == prev) { // if root->right is nonexistent or visited, visit root
                ans.push_back(root->val);
                s.pop();
                prev = root;
                root = nullptr;
            } else root = root->right; // otherwise, visit right subtree
        }
        return ans;
    }
};

See also 590. N-ary Tree Postorder Traversal (Easy)

Level-order

102. Binary Tree Level Order Traversal (Medium)

See more my solutions here

// OJ: https://leetcode.com/problems/binary-tree-level-order-traversal/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        if (!root) return {};
        vector<vector<int>> ans;
        queue<TreeNode*> q;
        q.push(root);
        while (q.size()) {
            int cnt = q.size();
            ans.emplace_back();
            while (cnt--) {
                root = q.front();
                q.pop();
                ans.back().push_back(root->val);
                if (root->left) q.push(root->left);
                if (root->right) q.push(root->right);
            }
        }
        return ans;
    }
};

• 2641. Cousins in Binary Tree II (Medium)

Other variations

• 107. Binary Tree Level Order Traversal II (Easy)

• 103. Binary Tree Zigzag Level Order Traversal (Medium)

Problems

• 124. Binary Tree Maximum Path Sum (Hard)