[数据结构与算法]leetcode-左叶子之和

计算给定二叉树的所有左叶子之和。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    /**
    思路：进行前序遍历，找出叶子节点累加
     */
    int sum = 0;//和要定义在外面，以为要递归 
    public int sumOfLeftLeaves(TreeNode root) {
        //终止条件
        if(root == null) return 0;
        //左叶子节点（度为0的叶子节点）
        if(root.left != null && root.left.left == null && root.left.right == null){
            sum += root.left.val;
        }
        //进行递归
        sumOfLeftLeaves(root.left);
        sumOfLeftLeaves(root.right);

        return sum;

    }

}