687. Longest Univalue Path

solution: DFS O(N), O(N)

class Solution {
    int res = 0;
    public int longestUnivaluePath(TreeNode root) {
        if (root == null) {
            return 0;
        }
        dfs(root, root.val);
        return res;
    }

    // Returns the length of the longest path under the root that have the value same as the root. 
    // The path could either be on the left or right child of the root. 
    public int dfs(TreeNode node, int val) {
        if (node == null) {
            return 0;
        }

        int left = dfs(node.left, node.val);
        int right = dfs(node.right, node.val);
        
        // the result should be left + right
        res = Math.max(res, left + right); 

        if (node.val == val) {
            // only keep one side for higher level node
            return 1 + Math.max(left, right);
        }

        return 0;
    }
    // left + right is the length of a path that goes from left subtree and through the current root to its right subtree. 
    // And from current root, we only return the longer branch between left and right.
}