[LeetCode] DFS-BFS 124. Binary Tree Maximum Path Sum

문제

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.

 

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.

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.

 

Constraints:

• The number of nodes in the tree is in the range [1, 3 * 104].
• -1000 <= Node.val <= 1000

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문제풀이

- 전체 합 혹은 계산 => DFS로 풀기

- left, right가 있는 그 path들의 모든 합 중 => 가장 적절한 path

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코드

// DFS

var maxPathSum = function(root) {
    let maxVal = -Infinity
    const traverse  = (root) => {
        if (!root) return 0
        const left = traverse(root.left)
        const right = traverse(root.right)
        maxVal = Math.max(left + right + root.val, maxVal)
        return Math.max(left + root.val, right + root.val,0)
    }
    traverse(root)
    return maxVal;
};