已知PA垂直于正方形ABCD所在的平面，M，N分别在AB，PC上，且PN=2NC，AM=2MB，PA=AD=1，如图建立空间直角坐际系，求$\overrightarrow{MN}$的坐标．

    A young man wandered through the desert for forty until he reached a beautiful castle at the top of a mountain. There lived the sage that he was looking for.

    With considerable patience, the sage listened attentively to the reason for the boy's visit, but told him that at that moment he did not have time to explain to him the secret of happiness.

    He suggested that the young man take a walk around his palace and come back in two hours' time.

    “However, I want to ask you a favor,” he added, handing the boy a teaspoon, in which he poured two drops of oil. “While you walk, carry this spoon and don't let the oil spill.”

    The young man began to climb up and down the palace staircases, always keeping his eyes fixed on the spoon. At the end of two hours he returned to the presence of the wise man.

    “So,” asked the sage, “did you see the Persian tapestries hanging in my dining room? Did you see the fantastic garden that the Master of Gardeners spent ten years in creating?”

    Embarrassed, the young man admitted that he had seen nothing.

    “So, go back and see the wonders of my world,” said the wise man. “You can't trust a man if you don't know his house.”

    Now more at ease, the young man took the spoon and wandered again through the palace, this time paying attention to all the works of art that hung from the ceilings and walls. He saw the gardens, the mountains all around the palace, and the delicacy of the flowers. Returning to the sage, he reported in detail what he had seen.

    “But where are the two drops of oil that I entrusted to you?” asked the Sage.

    Looking down at the spoon, the young man realized that he had spilled the oil.

     “Well, that is the only advice I have to give you,'' said the sage of sages. “The secret of happiness lies in looking at all the wonders of the world and never forgetting the two drops of oil in the spoon.”