To complement the answer from user3376.
Firstly, in Ezra 7, Ezra himself enters the story, and chooses to call the king, Artaxerxes, 'he who reigns in truth'. And just before the memoed Aramaic script (Ezra 4:8-Ezra 6:18) a king called Xerxes, 'king of kings'.
[Elsewhere, in Daniel 9:1, the title Xerxes, possibly referring to Astyages, is used - 'Darius, son of Xerxes']
This title 'king of kings' is reserved for Jesus himself (not a pretender), so in effect Ezra is showing us an image of the true king:
On his robe and on his thigh he has this name written: king of kings and lord of lords. Revelation 19:16
My answer therefore assumes that the answer to the question 'Who is the Artaxerxes of Ezra 7?' is Darius I - my rationale is given here.
Just before Ezra reveals just why he uses this title, by calling Darius by another curious title - king of Assyria.
For seven days they celebrated with joy the Festival of Unleavened Bread, because the Lord had filled them with joy by changing the attitude of the king of Assyria so that he assisted them in the work on the house of God, the God of Israel. Ezra 6:22
You need only look back as far as the decisive events that brought about the exile to Babylon to see how Ezra might have understood the turn around where they were now greeted with ‘a changing attitude’ in the reign of Darius I.
The ‘king of Assyria’ describes the battle for the centre or high ground by the contending empires. One could consider it to be a more serious form of the childrens’ game ‘King of the castle’ throughout time.
The centre ground is the around the middle of the fertile crescent, here the battles of the empires meet. It is in the prophetic words of Ezekiel that we see Assyria allegorically placed in Lebanon.
Like an actual cedar called something like Assyria, 'teasshur', (a play on words I presume), Assyria’s pride was brought down by God (cedars once covered a larger are in antiquity):
I gave it into the hands of the ruler of the nations, for him to deal with according to its wickedness. I cast it aside, and the most ruthless of foreign nations cut it down and left it. Its boughs fell on the mountains and in all the valleys; its branches lay broken in all the ravines of the land. Ezekiel 31:11-12
At this time was the Battle of Megiddo where Judah’s king Josiah died in 609 BC at the hands of the Egyptians as they went on to join the fading Assyrian empire – where they were defeated at the Battle of Carchemish (northern Syria) by the Babylonians.
Ezekiel 31 was written to show how Egypt also would be brought down (Ezekiel 31:18).
Ezekiel 17 too, plays an allegory of a cedar. Focusing on events around the same time; when Judah’s last king is captured, and a small remnant is taken of to Babylon. But ends with the flourishing returnees under the Persian protection.
This is what the Sovereign Lord says: I myself will take a shoot from the very top of a cedar and plant it; I will break off a tender sprig from its topmost shoots and plant it on a high and lofty mountain. On the mountain heights of Israel, I will plant it; it will produce branches and bear fruit and become a splendid cedar. Birds of every kind will nest in it; they will find shelter in the shade of its branches. All the trees of the forest will know that I the Lord bring down the tall tree and make the low tree grow tall. I dry up the green tree and make the dry tree flourish. Ezekiel 17:22-24
In this sense Ezra is indeed seeing the low ‘Assyria’ tree grow tall, giving the assistance build the temple.
For seven days they celebrated with joy the Festival of Unleavened Bread, because the Lord had filled them with joy by changing the attitude of the king of Assyria so that he assisted them in the work on the house of God, the God of Israel. Ezra 6:22