404. Sum of Left Leaves （二叉树中左叶子值之和）

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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        if(root==null)
        return 0;
        if(root.left==null)
        return sumOfLeftLeaves(root.right);
        if(root.left.left==null&&root.left.right==null)
        return root.left.val+sumOfLeftLeaves(root.right);
        return sumOfLeftLeaves(root.left)+sumOfLeftLeaves(root.right);
    }
}