43 lines
829 B
Rust
43 lines
829 B
Rust
struct Solution {}
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impl Solution {
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pub fn pivot_integer(n: i32) -> i32 {
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// S_i = S_n - S_{i - 1}
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// i * (1 + i) / 2 = n * (1 + n) / 2 - i * (i - 1) / 2
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// i * (1 + i) = n * (1 + n) - i * (i - 1)
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// 2i² = n(1 + n)
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// i = sqrt(n * (1 + n) / 2)
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let expected_ii = n * (1 + n) / 2;
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let mut i = 1;
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while i * i < expected_ii {
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i += 1;
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}
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if i * i == expected_ii {
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return i;
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}
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-1
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn example_1() {
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assert_eq!(Solution::pivot_integer(8), 6);
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}
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#[test]
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fn example_2() {
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assert_eq!(Solution::pivot_integer(1), 1);
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}
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#[test]
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fn example_3() {
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assert_eq!(Solution::pivot_integer(4), -1);
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}
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}
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