LeetCode 404. 左叶子之和(Sum of Left Leaves)

404. 左叶子之和

404. Sum of Left Leaves

LeetCode404. Sum of Left Leaves

题目描述

计算给定二叉树的所有左叶子之和。

示例:

    3
   / \
  9  20
    /  \
   15   7

在这个二叉树中，有两个左叶子，分别是 9 和 15，所以返回 24。

Java 实现

TreeNode 结构

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

Recursive

class Solution {
    private int sum = 0;

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

class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        int count = 0;
        if (root == null) {
            return 0;
        }
        if (root.left != null) {
            if (root.left.left == null && root.left.right == null) {
                count += root.left.val;
            } else {
                count += sumOfLeftLeaves(root.left);
            }
        }
        count += sumOfLeftLeaves(root.right);
        return count;
    }
}

Iterative

import java.util.Stack;
class Solution {
    public int sumOfLeftLeaves(TreeNode root) {
        int count = 0;
        Stack<TreeNode> stack = new Stack<>();
        if (root == null) {
            return 0;
        }
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.left != null) {
                if (node.left.left == null && node.left.right == null) {
                    count += node.left.val;
                } else {
                    stack.push(node.left);
                }
            }
            if (node.right != null) {
                if (node.right.left != null || node.right.right != null) {
                    stack.push(node.right);
                }
            }
        }
        return count;
    }
}

主测试类

public class Test {
    public static void main(String[] args) {
        Solution tree = new Solution();
        /* create a tree */
        TreeNode root = new TreeNode(3);
        root.left = new TreeNode(9);
        root.right = new TreeNode(20);
        root.right.left = new TreeNode(15);
        root.right.right = new TreeNode(7);
        System.out.println(tree.sumOfLeftLeaves(root));
    }
}

运行结果

24

相似题目

• 求二叉树中叶子节点的个数

参考资料

• https://leetcode.com/problems/sum-of-left-leaves/
• https://leetcode-cn.com/problems/sum-of-left-leaves/