LeetCode 404. Sum of Left Leaves

Find the sum of all left leaves in a given binary tree.

Example:

    3
   / 
  9  20
    /  
   15   7

There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

题意：给定一颗二叉树，返回该二叉树的左叶子结点的和
思路：层次遍历，然后判断每个结点是否包含左叶子结点，如果存在左叶子结点则加进sum，代码如下：

public int sumOfLeftLeaves(TreeNode root) {
        if(root == null)
            return 0;
        int sum = 0;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()){
            TreeNode t = q.poll();
            sum += isLeftLeaf(t);
            if(t.left != null){
                q.offer(t.left);
            }
            if(t.right != null){
                q.offer(t.right);
            }
        }
        return sum;
    }
    public int isLeftLeaf(TreeNode t){
        if(t.left != null){
            if(t.left.left == null && t.left.right == null)
               return t.left.val;
        }
        return 0;
    }