[LeetCode] 404. Sum of Left Leaves

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题：https://leetcode.com/problems/sum-of-left-leaves/description/

题目

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.

题目大意

求所有左叶子的和。其中左叶子的定义为：1.为叶子；2.为某结点的左子树。

思路

先序遍历，若一个结点为左子树为 叶子，那么累加。

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

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