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iterator.js

var dfs = {
  a: 1,
  b: {
    c: 2,
    d: 3,
    e: 4,
    f: {
      g: 5,
      h: 6
    },
    i: 7
  },
  j: {
    k: 8,
    l: 9,
    m: {
      n: {
        o: {
          p: 10
        }
      },
      q: 11
    },
    r: 12,
    s: 13
  },
  t: 14,
  u: 15,
  v: {
    w: 16,
    x: 17,
    y: {
      z: 18
    }
  }
};


var Iterator = function (tree) {
  this.tree = tree;
  this.flatArray = [];
  this.ct = -1;
};

Iterator.prototype.flatten = function (tree) {
  tree = tree || this.tree;
  Object.keys(tree).forEach(function (node) {
    var obj = {};

    if (typeof tree[node] === "object") {
      this.flatten(tree[node]);
    } else {
      obj[node] = tree[node];
      this.flatArray.push(obj);
    }
  }.bind(this));
};

Iterator.prototype.next = function () {
  if (this.flatArray.length === 0) {
    this.flatten();
  }

  this.ct += 1;

  return this.ct < this.flatArray.length ? this.flatArray[this.ct] : null;
};

var tree = new Iterator(dfs);
for (var i = 0; i < 25; i += 1) {
  console.log(tree.next());
}