problem of install opencv-android on ubuntu 12.04 64 bit

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i do the install according to the tutoiral of stanford (http://www.stanford.edu/class/ee368/Android/Tutorial-2-OpenCV-for-Android-Setup-Linux-API11.pdf),but in the final step, i have a problem: Android NDK: Could not find platform files (headers and libraries) /home/dli/programme/android-ndk-r9/build/core/init.mk:471: * Android NDK: Aborting . Stop. Android NDK: Please define NDK_PLATFORMS_ROOT to point to a valid directory.

can someone here help me?