LeetCode 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.

题意：求给定二叉树的最小深度

思路1：层次遍历，找到最近的叶子结点就返回层数。代码如下：

public int minDepth(TreeNode root) {
        if (root == null)
            return 0;
        int count = 1, size;
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        while (!q.isEmpty()) {
            size = q.size();
            for (int i = 0; i < size; i++) {
                TreeNode t = q.poll();
                if (t.left == null && t.right == null)
                    return count;
                if (t.left != null)
                    q.add(t.left);
                if (t.right != null)
                    q.add(t.right);
            }
            count++;
        }
        return 0;
    }

思路2：利用递归

public int minDepth(TreeNode root) {
        if (root == null)
            return 0;
        if (root.left == null && root.right == null) // 如果当前结点的左右子树都为空，则当前结点为叶子结点
            return 1;
        if (root.left == null)// 左子树为空，右子树非空
            return 1 + minDepth(root.right);
        else if (root.right == null)// 左子树非空，右子树为空
            return 1 + minDepth(root.left);
        else // 左右子树都非空
            return 1 + Math.min(minDepth(root.left), minDepth(root.right));
    }