题解 | #牛群左侧视图#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */
    public int[] leftSideView (TreeNode root) {
        // write code here
        List<Integer> result = new ArrayList<>();
        if(root == null){
            return new int[]{};
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while(!queue.isEmpty()){
            TreeNode node = queue.poll();
            result.add(node.val);
            if(node.left != null){
                queue.add(node.left);
            }else if(node.right != null){
                queue.add(node.right);
            }
        }

        int[] arr = new int[result.size()];      
        for(int i = 0;i < result.size();i++){
            arr[i] = result.get(i);
        }
        return arr;
    }
}