404. Sum of Left Leaves

原题网址：https://leetcode.com/problems/sum-of-left-leaves/

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.

方法：递归，深度遍历搜索。

/** * 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) {        int sum = 0;        if (root == null) return sum;        if (root.left != null) {            if (root.left.left == null && root.left.right == null) {                sum += root.left.val;            } else {            sum += sumOfLeftLeaves(root.left);            }        }        if (root.right != null) {            sum += sumOfLeftLeaves(root.right);        }        return sum;    }}