543.Diameter of Binary Tree¶

543.Diameter of Binary Tree¶

Tags: Easy Tree

Company: Facebook-34, Amazon-13, Microsoft, 字节跳动

Year: 半年内

Links: https://leetcode.com/problems/diameter-of-binary-tree/

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example: Given a binary tree

Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

Note: The length of path between two nodes is represented by the number of edges between them.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    unordered_map<TreeNode*, int> um;

public:
    int diameterOfBinaryTree(TreeNode* root) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        if (!root) return 0;

        int leftDepth = 0;
        if (root -> left) leftDepth = 1 + maxDepth(root -> left);
        int rightDepth = 0;
        if (root -> right) rightDepth = 1 + maxDepth(root -> right);

        return max(leftDepth + rightDepth, max(diameterOfBinaryTree(root -> left), diameterOfBinaryTree(root -> right)));
    }

    int maxDepth(TreeNode * root)
    {
        if (um.find(root) != um.end()) return um[root];

        if (!root -> left && !root -> right) {
            um[root] = 0;
            return 0;
        }

        int l = 0, r = 0;
        if (root -> left) l = maxDepth(root -> left);
        if (root -> right) r = maxDepth(root -> right);
        um[root] = 1 + max(l, r);

        return um[root];
    }
};

左子树的最大深度加上右子树的最大深度，然后和左右子树的最大直径比较，用了两次递归，用unordered_map来存储记录，减少重复计算。

当然也可以只进行一次递归：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    unordered_map<TreeNode*, int> um;
    int res;
public:
    int diameterOfBinaryTree(TreeNode* root) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        maxDepth(root);
        return res;
    }

    int maxDepth(TreeNode * root)
    {
        if (!root) return 0;
        if (um.find(root) != um.end()) return um[root];

        int l = maxDepth(root -> left);
        int r = maxDepth(root -> right);
        res = max(res, l + r);
        return (um[root] = 1 + max(l, r));
    }
};