63--binary-tree-maximum-path-sum.java

class Solution {
    int max = Integer.MIN_VALUE;
    public int maxPathSum(TreeNode root) {
        dfs(root);
        return max;
    }
    public int dfs(TreeNode root) {
        if (root == null) return 0;
        int leftMax = Math.max(0, dfs(root.left));
        int rightMax = Math.max(0, dfs(root.right));
        max = Math.max(max, root.val + leftMax + rightMax);
        return root.val + Math.max(leftMax, rightMax);
    }
} // TC: O(n), SC: O(n)