199. Binary Tree Right Side View

Given the root of a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

 

Example 1:

Input: root = [1,2,3,null,5,null,4]
Output: [1,3,4]

public List<Integer> rightSideView(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();
    helper(root, result, 0);
    return result;
}

public void helper(TreeNode curr, List<Integer> result, int currDepth) {
    if (curr == null) {
        return;
    }
    if (currDepth == result.size()) {
        result.add(curr.val);
    }
    helper(curr.right, result, currDepth + 1);
    helper(curr.left, result, currDepth + 1); // 对左子树也需要遍历，eg：跟节点只有个左子树，那么左子树也要加到list中
}