Easy and simple to understand. Create a helper recursive method like you do to compute sum of all leaves. However, in this case, keep a flag "isLeftChild", which helps in determining whether the node is a left child or not. By default, leetcode does not consider root as a left child.

public int sumOfLeftLeaves(TreeNode root) { // root is not considered as a left child return helper(root, false); } int helper(TreeNode root, boolean isLeftChild { if (root == null) return 0; // if current node is leaf, and it is a left child, return its value if (root.left == null && root.right==null) { if (isLeftChild) { return root.val; } else return 0; } return helper(root.left, true) + helper(root.right, false); }Sum of Left Leaves