LeetCode Hot 100 P543 Diameter of Binary Tree

---

Timeline

Timeline

2026-03-14

init

Post-order traversal, depth of binary tree

---

Problem:

P543 Diameter of Binary Tree

https://leetcode.cn/problems/diameter-of-binary-tree/description/?envType=study-plan-v2&envId=top-100-liked

Iterative approach:

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)
        {
        }
};

#include <stack>
#include <unordered_map>

using std::stack;
using std::unordered_map;

class Solution {
    public:
        int diameterOfBinaryTree(TreeNode *root)
        {
                // For each node, the maximum of left subtree depth + right subtree depth
                stack<TreeNode *> stk;
                unordered_map<TreeNode *, int> node_depth;
                TreeNode *last = nullptr, *p = root;
                int res = 0, left_depth, right_depth;

                while (p || !stk.empty()) {
                        if (p) {
                                stk.push(p);
                                p = p->left;
                        } else {
                                p = stk.top();
                                if (p->right && last != p->right) {
                                        p = p->right;
                                } else {
                                        // visit p

                                        left_depth = (p->left) ? node_depth[p->left] : 0;

                                        right_depth = (p->right) ? node_depth[p->right] : 0;

                                        node_depth[p] = std::max(left_depth, right_depth) + 1;

                                        res = std::max(res, left_depth + right_depth);

                                        stk.pop();

                                        last = p;
                                        p = nullptr;
                                }
                        }
                }

                return res;
        }
};

Recursive approach: actually it is to find the maximum of the sum of left and right subtree depths for each node

class Solution {
        int ans;
        int depth(TreeNode *rt)
        {
                if (rt == NULL) {
                        return 0; // Reached a null node, return 0
                }
                int L = depth(rt->left); // Depth of the subtree rooted at the left child
                int R = depth(rt->right); // Depth of the subtree rooted at the right child
                ans = max(ans, L + R + 1); // Compute d_node = L + R + 1 and update ans
                return max(L, R) + 1; // Return the depth of the subtree rooted at this node
        }

    public:
        int diameterOfBinaryTree(TreeNode *root)
        {
                ans = 1;
                depth(root);
                return ans - 1;
        }
};