自顶向下解决方法”Top-down” Solution

例如找一颗二叉树的最大深度：

private int answer;     // don't forget to initialize answer before call maximum_depth
private void maximum_depth(TreeNode root, int depth) {
    if (root == null) {
        return;
    }
    if (root.left == null && root.right == null) {
        answer = Math.max(answer, depth);
    }
    maximum_depth(root.left, depth + 1);
    maximum_depth(root.right, depth + 1);
}

自底向上解决方法”Bottom-up” Solution

例如找一颗二叉树的最大深度：

public int maximum_depth(TreeNode root) {
    if (root == null) {
        return 0;                                   // return 0 for null node
    }
    int left_depth = maximum_depth(root.left);
    int right_depth = maximum_depth(root.right);
    return Math.max(left_depth, right_depth) + 1;   // return depth of the subtree rooted at root
}