题解 | #牛群左侧视图#

知识点：层序遍历

思路：层序遍历，无非是层序遍历过程中的各种数据改编式处理，不多说了

编程语言：java

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
        if(root == null)
            return new int[0];
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        List<Integer> list = new ArrayList<>();
        while (!queue.isEmpty()){
            int size = queue.size();
            int i=0;
            int start=-1;
            boolean flag = true;
            for (i=0;i<size;i++){
                if(queue.peek().left != null)
                    queue.add(queue.peek().left);
                if(queue.peek().right != null)
                    queue.add(queue.peek().right);
                if(flag){
                    start = queue.peek().val;
                    flag = !flag;
                }
                queue.poll();
            }
            list.add(start);
        }
        return list.stream().mapToInt(Integer::intValue).toArray();
    }
}