543. Diameter of Binary Tree

class Solution {
    private static class State {
        private int maxLength = 0;
    }

    public int diameterOfBinaryTree(TreeNode root) {
        State state = new State();
        dfs(root, state);
        return state.maxLength;
    }

    private int dfs(TreeNode root, State state) {
        if (root == null) {
            return 0;
        }

        int leftHeight = dfs(root.left, state);
        int rightHeight = dfs(root.right, state);
        state.maxLength = Math.max(state.maxLength, leftHeight + rightHeight);
        return Math.max(leftHeight, rightHeight) + 1;
    }
}
Reference
