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513. 找树左下角的值

使用层序遍历，找到最后一层时输出即可

/*** 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 {public int findBottomLeftValue(TreeNode root) {Deque<TreeNode> q=new LinkedList<>();q.add(root);TreeNode last=root;int sum=0;TreeNode head=root;while(!q.isEmpty()){TreeNode cur=q.poll();if(cur.left!=null){q.add(cur.left);sum++;}if(cur.right!=null){q.add(cur.right);sum++;}if(cur==last){if(sum==0){return head.val;}sum=0;head=q.peekFirst();last=q.peekLast();}}return 0;}
}