Binary Tree Maximum Path Sum - yokolet's notelets

Problem Description

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.

The path sum of a path is the sum of the node’s values in the path.

Given the root of a binary tree, return the maximum path sum of any non-empty path.

Constraints:

The number of nodes in the tree is in the range [1, 3 * 10**4].

-1000 <= Node.val <= 1000

https://leetcode.com/problems/binary-tree-maximum-path-sum/

Examples

Example 1
Input: root = [1,2,3]
Output: 6
Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6.
   1
 /   \
2     3

Example 2
Input: root = [-10,9,20,null,null,15,7]
Output: 42
Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.
    -10
  /     \
9       20
       /  \
     15    7

How to Solve

Do postorder (DFS) traversal and calculate the value, The node value + left subtree value + right subtree value, which is the sum at the current root. Compare the value with the max value so far. When returning the value, considerable paths are: root + left subtree, root + right subtree, or 0 (doesn’t take this subtree). Return the max value of those three.

What needs to be care about is, how to save max value so far. Python solution pass it as a method argument and returns it. Ruby solution uses an instance variable. C++ solution passes a reference.

Solution

using namespace std;

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) {}
};

class BinaryTreeMaximumPathSum {
private:
    int traverse(TreeNode *root, int &max_v) {
        if (!root) { return 0; }
        int left = max(traverse(root->left, max_v), 0);
        int right = max(traverse(root->right, max_v), 0);
        max_v = max(max_v, root->val + left + right);
        return root->val + max(left, right);
    }

public:
    int maxPathSum(TreeNode* root) {
        int max_v = INT_MIN;
        traverse(root, max_v);
        return max_v;
    }
};

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    private int max_v = Integer.MIN_VALUE;
    private int traverse(TreeNode root) {
        if (root == null) { return 0; }
        int left = Math.max(traverse(root.left), 0);
        int right = Math.max(traverse(root.right), 0);
        max_v = Math.max(max_v, root.val + left + right);
        return root.val + Math.max(left, right);
    }

    public int maxPathSum(TreeNode root) {
        traverse(root);
        return max_v;
    }
}

function TreeNode(val, left, right) {
  this.val = (val===undefined ? 0 : val)
  this.left = (left===undefined ? null : left)
  this.right = (right===undefined ? null : right)
}

/**
 * @param {TreeNode} root
 * @return {number}
 */
var maxPathSum = function(root) {
  let max_v = Number.MIN_SAFE_INTEGER;
  const traverse = (root) => {
    if (root === null) { return 0; }
    const left = Math.max(traverse(root.left), 0);
    const right = Math.max(traverse(root.right), 0);
    max_v = Math.max(max_v, root.val + left + right);
    return root.val + Math.max(left, right)
  };
  traverse(root);
  return max_v;
};

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
from typing import Optional

class BinaryTreeMaximumPathSum:
    def maxPathSum(self, root: Optional[TreeNode]) -> int:
        def traverse(root: Optional[TreeNode], max_v: int) -> (int, int):
            if not root: return max_v, 0
            max_v, left = traverse(root.left, max_v)
            max_v, right = traverse(root.right, max_v)
            max_v = max(max_v, root.val + left + right)
            return max_v, max(root.val + max(left, right), 0)
        max_v, _ = traverse(root, float('-inf'))
        return max_v

# Definition for a binary tree node.
# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val = 0, left = nil, right = nil)
#         @val = val
#         @left = left
#         @right = right
#     end
# end
# @param {TreeNode} root
# @return {Integer}
def max_path_sum(root)
    @max_value = -Float::INFINITY
    traverse = -> (root) {
      return 0 if root.nil?
      l_max = [traverse.call(root.left), 0].max
      r_max = [traverse.call(root.right), 0].max
      @max_value = [@max_value, root.val + l_max + r_max].max
      return [root.val, root.val + [l_max, r_max].max].max
    }
    traverse.call(root)
    @max_value
end

Complexities

Time: O(n)
Space: O(h) – h: height of the binary tree