Write The Shortest Program To Check If A Binary Tree Is Balanced

Jelly, 11 bytes

ḊµŒḊ€IỊ;߀Ạ

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The empty tree is represented by [].

Prolog (SWI), 49 bytes

N+_/B/C:-X+B,Y+C,abs(X-Y)<2,N is max(X,Y)+1.
0+e.

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Represents trees as Value/Left_Child/Right_Child, with the empty tree being the atom e. Defines +/2, which outputs through success or failure, with an unbound variable (or one already equal to the tree's height) on the left and the tree on the right--if the height argument is unacceptable, add 9 bytes to define -T:-_+T..

N + _/B/C :-            % If the second argument is a tree of the form _Value/B/C,
    X+B,                % X is the height of its left child which is balanced,
    Y+C,                % Y is the height of its right child which is balanced,
    abs(X-Y) < 2,       % the absolute difference between X and Y is strictly less than 2,
    N is max(X,Y)+1.    % and N is the height of the full tree.
0 + e.                  % If, on the other hand, the second argument is e, the first is 0.

JavaScript (Node.js), 49 bytes

h=([l,r])=>l?(d=h(l)-h(r))*d<2?1+h(d>0?l:r):NaN:1

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-9 bytes by Arnauld.

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JavaScript, 58 bytes

h=([l,r])=>l?(l=h(l),r=h(r),m=l>r?l:r,m+m-l-r<2?m+1:NaN):1

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Use [] for null, and [left, right, value] for nodes.