124. Binary Tree Maximum Path Sum（js）

124. Binary Tree Maximum Path Sum

Given a non-empty binary tree, find the maximum path sum.

For this problem, a path is defined as any sequence of nodes from some starting node to any node in the tree along the parent-child connections. The path must contain at least one node and does not need to go through the root.

Example 1:

Input: [1,2,3]

       1
      / \
     2   3

Output: 6

Example 2:

Input: [-10,9,20,null,null,15,7]

   -10
   / \
  9  20
    /  \
   15   7

Output: 42
题意：求二叉树最大路径和
代码如下：

/*
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var maxPathSum = function(root) {
    let res=-Infinity;
    function sum(root){
        if(!root) return 0;
        let left=sum(root.left);
        let right=sum(root.right);
        if(left>0 && right>0){
            res=Math.max(res,left+right+root.val);
        }else if(left>0){
            res=Math.max(res,left+root.val);
        }else if(right>0){
            res=Math.max(res,right+root.val);
        }else{
            res=Math.max(res,root.val);
        }
        return Math.max(root.val,root.val+Math.max(left,right));
    }
    sum(root);
    return res;
}