Sum of Left Leaves (Leetcode 404)

• Leetcode 404
• Video Walkthrough

Given the root of a binary tree, return the sum of all left leaves.

A leaf is a node with no children. A left leaf is a leaf that is the left child of another node.

Examples

• Example 1
  • Input
    • root = [3,9,20,null,null,15,7]
  • Output: 24
  • Explanation
    • There are two left leaves in the binary tree, with values 9 and 15 respectively
• Example 2
  • Input
    • root = [1]
  • Output: 0

Solution

const sumOfLeftLeaves = (root) => {
  let sum = 0;

  const checkNode = (root) => {
    if (root === null) {
      return;
    }

    // if node has left and left has no children (is leaf node) then add value to sum
    if (root.left && !root.left.left && !root.left.right) {
      sum = sum + root.left.val;
    }

    // continue to check child nodes
    checkNode(root.left);
    checkNode(root.right);
  };

  checkNode(root);
  return sum;
};

// [3,9,20,null,null,15,7]
const example1 = {
  val: 3,
  left: { val: 9, left: null, right: null },
  right: {
    val: 20,
    left: { val: 15, left: null, right: null },
    right: { val: 7, left: null, right: null }
  }
};

//[1]
const example2 = { val: 1, left: null, right: null };

// [1,2,3,4,null,null,5]
const example3 = {
  val: 1,
  left: { val: 2, left: { val: 4, left: null, right: null }, right: null },
  right: { val: 3, left: null, right: { val: 5, left: null, right: null } }
};

console.log("example1", sumOfLeftLeaves(example1));
console.log("example2", sumOfLeftLeaves(example2));
console.log("example3", sumOfLeftLeaves(example3));