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Find the sum of all left leaves in a given binary tree.
Example:
3 / \ 9 20 / \ 15 7There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.

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public class Solution {
    /*Recursive method. For given node we check whether its left child is a leaf. If it is the case, we add its value to answer, otherwise recursively call method on left child. For right child we call method only if it has at least one nonnull child.
    */
    public int sumOfLeftLeaves(TreeNode root) {
        if (root == null) return 0;
        int ans = 0;
        
        if (root.left != null) { 
            if (root.left.left == null && root.left.right == null) {
                ans += root.left.val;
            } else {
                ans += sumOfLeftLeaves(root.left);
            }
        } 
        if (root.right != null) {
            ans += sumOfLeftLeaves(root.right);
        }
        return ans;
    }
}