findChildNode.js

const findChildNode = function(array) {
  const index = {};
  let target;
  for (let i = 0; i < array.length; i++) {
    const parentNode = array[i][0];
    const childNode = array[i][1];
    if (index[childNode]) {
      index[childNode].push(parentNode);
    } else {
      index[childNode] = [parentNode];
    }
  }

  for (let childNode in index) {
    if (index[childNode].length == 1) {
      target = childNode;
    }
  }

  return target;
}

findChildNode([[0, 1],[1, 3],[2, 3],[3, 5],[5, 9],[4, 5],[7, 1]]);