class Solution(object):
    def findComplement(self, num):
        i=1
        while i<=num:
            i = i << 1
        return (i-1)^num
# num: 5 = 101;
# i=1, after i<<1: shift left, i=10;
# i=100,i=1000; now i>num;
# 1000-1= 111; 111^101= 010 got it!