题目：
404. Sum of Left Leaves(easy)
Find the sum of all left leaves in a given binary tree.
Example:1
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7 3
/ \
9 20
/ \
15 7
There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
题目大意：
求出给定二叉树的所有左叶子之和。
解题思路：
递归 + 判断左叶子.
代码更易理解。
代码：1
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13Java:
public int sumOfLeftLeaves(TreeNode root){
if(root == null) return 0;
if(isLeaf(root.left)) return root.left.val + sumOfLeftLeaves(root.right);
return sumOfLeftLeaves(root.left) + sumOfLeftLeaves(root.right);
//一直往最底下找，相当于一直往内层嵌入，然后递归回来时从最底层网上加
}
private boolean isLeaf(TreeNode node){
if(node == null) return false;
return node.left == null && node.right == null;
}
