Binary Tree Right Side View（FB）

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

For example: Given the following binary tree,

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

You should return[1, 3, 4].

分析

判断每层用那个值，有右值用右值，没有的话用左值。用i == ret.size()。记得先右后左的递归。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> ret = new ArrayList<>();
    public List<Integer> rightSideView(TreeNode root) {        
        if(root == null){
            return ret;
        }
        helper(root, 0);
        return ret;
    }
    void helper(TreeNode root, int i){
        if(i == ret.size()){
            ret.add(root.val);
        }
        if(root.right != null){
            helper(root.right, i + 1);
        }
        if(root.left != null){
            helper(root.left, i + 1);           
        }
    }
}

bfs and dfs

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    // public List<Integer> rightSideView(TreeNode root) {
//BFS注意模板
//         Queue<TreeNode> q = new LinkedList<>();
//         List<Integer> ret = new ArrayList<>();
//         if(root == null){
//             return ret;
//         }
//         q.offer(root);
//         while(!q.isEmpty()) {
//             List<TreeNode> level = new ArrayList<>();
//             int size = q.size();
//             for(int i = 0; i < size; i ++) {
//                 TreeNode node = q.poll();
//                 level.add(node);
//                 if(node.right != null)
//                     q.offer(node.right);
//                 if(node.left != null)
//                     q.offer(node.left);

//             }
//             ret.add(level.get(0).val);
//         }
//         return ret;


    // }
    public List<Integer> rightSideView(TreeNode root) {


        List<Integer> ret = new ArrayList<>();
        helper(root, ret, 0);
        return ret;

    }

    void helper(TreeNode root, List<Integer> ret, int depth) {
        if(root == null){
            return ;
        }
        if(depth == ret.size()) {
            ret.add(root.val);
        }
        helper(root.right, ret, depth + 1);
        helper(root.left, ret, depth + 1);
    }
}