Min Depth of Binary Tree

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
void dfs(TreeNode* node, int dep, int& res) {
    if(!node->left and !node->right) res = min(res,dep);
    if(node->left) dfs(node->left, dep + 1, res);
    if(node->right) dfs(node->right, dep + 1, res);
}
int Solution::minDepth(TreeNode* A) {
    if(!A) return 0;
    int res = INT_MAX;
    dfs(A,1,res);
    return res;
}