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

Example:

3   / \  9  20    /  \   15   7There 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
     
       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;    }