Find Minimum Depth of Binary Tree - The Coding Shala

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Find Minimum Depth of Binary Tree

Given a binary tree, find its minimum depth. The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3

   / \

  9  20

      /  \

   15   7

return its minimum depth = 2.

Find Minimum Depth of Binary Tree Java Program

Method 1:

Using recursion.

Java Code: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if(root==null) return 0;
        if(root.left==null) return 1+minDepth(root.right);
        if(root.right==null) return 1+minDepth(root.left);
        return 1+Math.min(minDepth(root.left), minDepth(root.right));
    }
}

Method 2:

Using level order traversal.

We do level-order traversal and increase depth according to the current level when meeting a node in which both children is null then return the depth.

Java Code: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if(root==null) return 0;
        int depth = 1;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i=0;i<size;i++){
                TreeNode curr = queue.poll();
                if(curr.left==null && curr.right==null) return depth;
                if(curr.left!=null) queue.offer(curr.left);
                if(curr.right!=null) queue.offer(curr.right);
            }
            depth++;
        }
        return depth;
    }
}

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