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var validArrangement = function (pairs) {
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  let graph = {};
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  let degrees = {}; // outdegree: positive, indegree: negative
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  for (var [x, y] of pairs) {
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    if (!graph[x]) graph[x] = [];
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    graph[x].push(y);
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    if (degrees[x] === undefined) degrees[x] = 0;
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    if (degrees[y] === undefined) degrees[y] = 0;
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    degrees[x]++;
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    degrees[y]--;
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  }
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  let start = pairs[0][0];
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  for (var [x] of pairs) {
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    if (degrees[x] === 1) start = x; // one extra outdegree
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  }
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  let ans = [];
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  dfs(start);
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  function dfs(node) {
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    while ((graph[node] || []).length) {
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      let neighbor = graph[node].pop();
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      dfs(neighbor);
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      ans.push([node, neighbor]);
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    }
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  }
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  return ans.reverse();
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};