[Algo] 646. Store Number Of Nodes In Left Subtree

Given a binary tree, count the number of nodes in each node’s left subtree, and store it in the numNodesLeft field.

Examples




1(6)

/          \

2(3)        3(0)

/      \

4(1)     5(0)

/        \        \

6(0)     7(0)   8(0)

The numNodesLeft is shown in parentheses.

/**
 * public class TreeNodeLeft {
 *   public int key;
 *   public TreeNodeLeft left;
 *   public TreeNodeLeft right;
 *   public int numNodesLeft;
 *   public TreeNodeLeft(int key) {
 *     this.key = key;
 *   }
 * }
 */
public class Solution {
  public void numNodesLeft(TreeNodeLeft root) {
    // Write your solution here
    helper(root);
  }

  private int helper(TreeNodeLeft root) {
    if (root == null) {
      return 0;
    }
    int left = helper(root.left);
    int right = helper(root.right);
    root.numNodesLeft = left;
    return 1 + left + right;
  }
}