prepend,append-Sequence
C# (.NET Core), 83, 80, 60 59 bytes



n=&gt;{int i=0;for(var t="";t!=n;)t=++i%2&lt;1?t+i:i+t;return i;}


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Takes the input as a string into a lambda function. 1-indexed. Returns the index of the value for truthy, or infitnitely loops for a "falsey"