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By summing the number of nodes across all levels, the total size of the tree is estimated. The video uses the finite geometric series example $2^0+2^1+\cdots+2^6=2^7-1$ to show that the sum is dominated by the highest-order term.

Conditions: Time cost is measured by the number of recursive calls.; An upper bound estimation using a perfect binary tree is adopted.; The sum starts from $2^0$ with a common ratio of 2.

By summing the number of nodes across all levels, the total size of the tree is estimated. The video uses the finite geometric series example $2^0+2^1+\cdots+2^6=2^7-1$ to show that the sum is dominated by the highest-order term.

Conditions: Time cost is measured by the number of recursive calls.; An upper bound estimation using a perfect binary tree is adopted.; The sum starts from $2^0$ with a common ratio of 2.