一:题目

二:上码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
          
   

    /**
        利用递归求出层数
        然后迭代求出最后一层的元素
     */
    int getCen(TreeNode root) {
          
   
        if (root == null) return 0;

        int left_len = getCen(root.left);
        int right_len = getCen(root.right);

        return Math.max(left_len+1,right_len+1);
    } 

    public int getAns(TreeNode root) {
          
   
      ArrayDeque<TreeNode> queue = new ArrayDeque<>();
      if (root != null) queue.add(root);
      int cen = getCen(root);

      while (!queue.isEmpty()) {
          
   
          int size = queue.size();
          cen--;

          while (size > 0) {
          
   
              TreeNode node = queue.poll();

              if (node.left != null) queue.add(node.left);
              if (node.right != null) queue.add(node.right); 

              if (cen == 0) return node.val;

              size--;
          }  
      }
    
      return 0;
    }


    

    public int findBottomLeftValue(TreeNode root) {
          
   
        int ans = getAns(root);
        return ans;
       
    }   
}