111. Minimum Depth of Binary Tree

和求maxDepth基本一样

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

保证树左右子树任意一边为null的时候，返回有高度的一边

if (left == 0 || right == 0)  return left + right + 1;

相当于

if (root.left == null || root.right  == null) {
    return (root.right == null) ? 1 + left : 1 + right;
}