404. Sum of Left Leaves

Description

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.

Solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * Traverse the binary tree with Depth-First Search (DFS) algorithm
 * @param {TreeNode} root
 * @return {number}
 */
var sumOfLeftLeaves = function(root) {
    "use strict";
    let sum = 0;
    function dfs(node) {
        if (node.left) {
            if (!node.left.left && !node.left.right) {
               sum += node.left.val;
            }
            dfs(node.left);
        }
        node.right && dfs(node.right);
    }
    root && dfs(root);
    return sum;
};