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Number of 1 Bits — Brian Kernighan
Count set bits (popcount). Brian Kernighan: n & (n-1) clears the lowest set bit; count how many operations until n=0.
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Counting Bits — DP with Bit Trick
Count set bits for all numbers 0..n. DP: dp[i] = dp[i >> 1] + (i & 1). Remove last bit and check if it was set.
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Reverse Bits — 32-Bit Reversal
Reverse bits of a 32-bit unsigned integer. Shift result left and input right 32 times, taking last bit each time.
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Missing Number — XOR or Gauss Formula
Find missing number in 0..n array. XOR approach: XOR all indices with all values, duplicate indices cancel. Or math: n*(n+1)/2 - sum.
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Sum of Two Integers — Bit Addition
Add two integers without + or - operators. Simulate: sum = a XOR b (no carry), carry = (a AND b) << 1. Repeat until carry = 0.
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