404. 左叶子之和

使用层序遍历，找到左叶子结点加入答案即可

/*** 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 sumOfLeftLeaves(TreeNode root) {int ans=0;if(root==null)return ans;Queue<TreeNode> q=new LinkedList<>();q.add(root);while(!q.isEmpty()){TreeNode cur=q.poll();TreeNode left=cur.left;TreeNode right=cur.right;if(left!=null){if(left.left==null&&left.right==null)ans+=left.val;q.add(left);}if(right!=null){q.add(right);}}return ans;}
}