MinimumDepthOfBinaryTree111.java

/**
 * 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.
 * 
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

public class MinimumDepthOfBinaryTree111 {
    public int minDepth(TreeNode root) {
        if (root == null) return 0;
        Queue<TreeNode> q = new LinkedList<>();
        int depth = 1;
        q.add(root);
        while (!q.isEmpty()) {
            int size = q.size();
            for (int i=0; i<size; i++) {
                TreeNode node = q.remove();
                boolean isLeaf = true;
                if (node.left != null) {
                    isLeaf = false;
                    q.add(node.left);
                }
                if (node.right != null) {
                    isLeaf = false;
                    q.add(node.right);
                }
                if (isLeaf) return depth;
            }
            depth++;
        }
        return depth;
    }
    
}