Problem

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 - Recursive

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

Solution - Iterative

class Solution {    public int sumOfLeftLeaves(TreeNode root) {        if (root == null) return 0;        int res = 0;        Deque
    
      stack = new ArrayDeque<>();        stack.push(root);        while (!stack.isEmpty()) {            TreeNode cur = stack.pop();            if (cur.left != null) {                if (cur.left.left == null && cur.left.right == null) {                    res += cur.left.val;                } else {                    stack.push(cur.left);                }            }            if (cur.right != null) {                if (cur.right.left != null || cur.right.right != null) {                    stack.push(cur.right);                }            }        }        return res;    }}