lk111.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode() {}
 * TreeNode(int val) { this.val = val; }
 * TreeNode(int val, TreeNode left, TreeNode right) {
 * this.val = val;
 * this.left = left;
 * this.right = right;
 * }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) return 0;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        int depth = 1;

        while (!q.isEmpty()) {
            int sz = q.size();
            for (int i = 0; i < sz; i++) {
                TreeNode cur = q.poll();
                if (cur.left == null && cur.right == null) {
                    return depth;
                }
                if (cur.left != null)
                    q.offer(cur.left);
                if (cur.right != null)
                    q.offer(cur.right);
            }
            depth++;
        }
        return depth;
    }
}