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.

Note: A leaf is a node with no children.

Example:

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

return its minimum depth = 2.

Solution: Recursion¶

Time complexity: O(n) Space complexity: O(n)

class Solution {
public:
  int minDepth(TreeNode* root) {
    if (!root) return 0;
    if (!root->left && !root->right) return 1;
    int l = root->left ? minDepth(root->left) : INT_MAX;
    int r = root->right ? minDepth(root->right) : INT_MAX;
    return min(l, r) + 1;
  }
};

Queue version

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.add(root);
        int count = 0;
        while (q.size() > 0) {
            int n = q.size();
            count ++;
            boolean end = false;
            for (int i = 0; i < n; i ++) {
                TreeNode node = q.remove();
                if (node.left == null && node.right == null) {
                    end = true;
                }
                if (node.left != null) {
                    q.add(node.left);
                }
                if (node.right != null) {
                    q.add(node.right);
                }
            }
            if (end == true) {
                break;
            }
        }
        return count;
    }
}