LeetCode.404.Sum of Left Leaves 二叉树的左叶子和

题目描述

404 Sum of Left Leaves https://leetcode-cn.com/problems/sum-of-left-leaves/

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.

解题过程

左叶子和，递归，左叶子和等于左右子树的左叶子和之和。 注意单根节点不是左叶子，因为他不是父节点的左叶子。 时间复杂度 O(n), 空间复杂度O(n)

public int sumOfLeftLeaves(TreeNode root) {
    return sumOfLeftLeaves(root, false);
}

private int sumOfLeftLeaves(TreeNode root, boolean left) {
    if (null == root) {
        return 0;
    }
    if (null == root.left && null == root.right && left) {
        return root.val;
    }
    return sumOfLeftLeaves(root.left, true) + sumOfLeftLeaves(root.right, false);
}

GitHub代码

algorithms/leetcode/leetcode/_404_SumOfLeftLeaves.java https://github.com/masikkk/algorithms/blob/master/leetcode/leetcode/_404_SumOfLeftLeaves.java