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
.

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Tree

public class Solution {

    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);
            }
        }
        ans += sumOfLeftLeaves(root.right);
        return ans;
    }

}