Big O
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// O(n) => linear time
// O(1) => constant time
//in js:
//break; => exit the current loop
// in java：
// break终止loop
//continue => 终止本次循环开始下一次循环
//rule2: drop constant number in O()
//rule3: be careful about if looping the same items; do not 2n => n
JavaScript
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