_111.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

import java.util.LinkedList;
import java.util.Queue;

/**
 * 111. 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.
 * */

public class _111 {
    /**
     * We can solve this problem using both BFS and DFS:
     * DFS is to visit every single root to leaf path and return the shortest one.
     * BFS is to visit every level and return whenever we find the first leaf node.
     */

    public static class DFSSolution {

        public int minDepth(TreeNode root) {
            if (root == null) return 0;
            int left = minDepth(root.left);
            int right = minDepth(root.right);
            if (left == 0) return right + 1;
            if (right == 0) return left + 1;
            return Math.min(left, right) + 1;
        }

    }

    public static class BFSSolution {

        public int minDepth_BFS(TreeNode root) {
            if (root == null) return 0;
            Queue<TreeNode> q = new LinkedList();
            q.offer(root);
            int level = 0;
            while (!q.isEmpty()) {
                level++;
                int size = q.size();
                for (int i = 0; i < size; i++) {
                    TreeNode curr = q.poll();
                    if (curr.left != null) q.offer(curr.left);
                    if (curr.right != null) q.offer(curr.right);
                    if (curr.left == null && curr.right == null) return level;
                }
            }
            return level;
        }
    }
}